'How do I spend the rest of my life without my daughter?'

For more than four months, Ronggao Zhang has walked to his missing daughter's apartment almost every day. At first, he stood outside, hoping she would show up one afternoon. But even after he was told she'd been kidnapped and was presumed dead, he's continued his routine.

"It brings peace and comfort to my heart," Zhang explained in Mandarin, through a translator.

This photo provided by her family shows Yingying Zhang. The 26-year-old visiting scholar at the University of Illinois at Urbana-Champaign, disappeared June 9, 2017. A former graduate student has been charged with kidnapping and killing her. Zhang's body has not been found. (Family Photo via AP)

His daughter, Yingying Zhang (ying ying zahng), a 26-year-old visiting scholar at the University of Illinois at Urbana-Champaign, disappeared June 9 on her way to sign an apartment lease. A former graduate student has been charged with kidnapping and killing her. Zhang's body has not been found.

A few days ago, Zhang's father made a final visit to the Orchard Downs apartments with his wife, 24-year-old son and daughter's boyfriend as they prepared to return to China. They arrived here after Zhang vanished, hopeful in the beginning that she'd be found alive. After authorities relayed the grim news, they decided to stay until her remains were found so they could take her home for a proper burial, in accordance with Chinese customs.

Now they plan to leave Sunday, reluctantly, without her. Zhang's mother is in fragile health — she broke down at the start of a recent court hearing — and there's no way of knowing when this cruel mystery will be solved. Each day they wait, in agony.

"We don't know where she is, and I don't know how to spend the rest of my life without my daughter," said Lifeng Ye, Zhang's mother, her face tear-stained and voice trembling as she spoke through a translator. "I can't really sleep well at night. ... I often dream of my daughter, and she's right there with me. I want to ask the mother of the suspect, please talk to her son and ask him what he did to my daughter. Where is she now? I want to know the answer."

This 2016 selfie photo provided by her family shows Yingying Zhang in a cap and gown for her graduate degree in environmental engineering from Peking University Shenzhen Graduate School. She chose the University of Illinois at Urbana-Champaign for its highly regarded agriculture program. Zhang had been doing research on crop photosynthesis and was to begin her doctoral work in September 2017. (Family Photo via AP)

Authorities have not said how Zhang died. Brendt Christensen, 28, was charged in July with abduction and then last month accused in a superseding indictment of kidnapping resulting in death "in an especially heinous, cruel or depraved manner, in that it involved torture or serious physical abuse to the victim." That carries the possibility of the death penalty. Christensen's lawyer declined comment.

Federal prosecutors claim that Zhang, who arrived on campus in April, had missed a bus and worried she was late to sign an apartment lease when Christensen lured her into his car. Surveillance video showed her getting into the front seat of a black Saturn Astra the FBI alleges was cleaned in a way to conceal evidence.

Audio surveillance captured Christensen talking about how he abducted Zhang and brought her back to his apartment, where she "fought and resisted" while he held her against her will, according to prosecutors. They contend he also talked about who makes an "ideal victim," but prosecutors would not identify whom Christensen was speaking with or the source of those conversations.

A federal complaint disclosed that Christensen used his phone in April to visit a fetish networking site online, viewing threads titled "perfect abduction fantasy" and "planning a kidnapping." Christensen, who earlier this year earned a master's degree in physics, appeared at a campus vigil for Zhang in June before he was arrested.

Zhang's disappearance has rippled far beyond this quiet central Illinois community, generating headlines and discussion in Chinese newspapers and social media sites about the American justice system, the capabilities of law enforcement, and whether scholars who study in the U.S. are safe.

Ronggao Zhang, left, and Lifeng Ye, display a photo them with their missing daughter, Yingying Zhang, in Urbana, Ind., Wednesday, Nov. 1, 2017. They had hoped to stay in the United States until the remains of their daughter were found. But after months of agony, they are ready to return to China, where they will wait for answers half a world away. (AP Photo/Michael Conroy)

Some 5,600 Chinese students attend the university here — more than any other college in the nation. The Urbana-Champaign area typically has no more than a few homicides a year.

Studying in America was a longtime dream for Zhang and "she loved her stay here," said her boyfriend, Xiaolin Hou, who was in touch with her every day via WeChat, a popular social network in China.

She chose Illinois for its highly regarded agriculture program. Zhang had been doing research on crop photosynthesis. She was to begin her doctoral work in September after having earned a graduate degree in environmental engineering from the prestigious Peking University Shenzhen Graduate School.

"She was very tough, strong, never afraid of hard work," her mother said, recalling how she was at the top of her class ever since she was a little girl. Zhang was a calming influence, too, when her parents expressed concern about her safety in America. "She always told me, 'Mom, don't worry about me. There are Chinese here, Americans here. But everyone is very nice here.'"

Zhang was a devoted daughter. At the same time every Saturday, she'd call her parents in Nanping, China. As a graduate research student, she used her meager savings to buy her family a cellphone, an air conditioner and a microwave oven. She planned to become a university professor and help support her father, a factory worker, and mother, a homemaker.

"She never hesitated, even for a moment, when others needed help," said her boyfriend, who put his doctoral studies on hold to join Zhang's family here. He said her selflessness was what attracted him to Zhang when they met in their first year in college. She also knew how to have fun, playing guitar and singing lead in a band called "Cute Horse."

Xiaolin Hou, foreground, the boyfriend Yingying Zhang, a missing University of Illinois at Urbana-Champaign student, talks about her disappearance, accompanied by her parents, Lifeng Ye, left, Ronggao Zhang, and her brother, Xinyang Zhang, in Urbana, Ind., Wednesday, Nov. 1, 2017. Studying in America was a longtime dream for Yingying and “she loved her stay here,” said Hou, who was in touch with her every day via WeChat, a popular social network in China. (AP Photo/Michael Conroy)

Hou and family members have met with the FBI, police and prosecutors, but they're frustrated by the slow pace of justice, said Zhidong Wang, a Chicago lawyer who has been helping them. He said he's explained that even though authorities have a suspect, Christensen's constitutional rights protect him from being forced to reveal anything that would hurt his defense.

Christensen's lawyers recently sought a delay in the trial until next October, saying they needed to check into several purported sightings of Zhang and reports of suspicious people around her apartment before she disappeared.

Zhang's father said the loss of his daughter has warped his sense of time: "Every day is like a year."

Hou, Zhang's boyfriend of eight years, has struggled, too. Though they didn't have marriage plans, he said, "In my heart, she is my wife for all time."

When authorities provided scanned pages of Zhang's diary to the family, Hou said it hurt too much to read them thoroughly. She wrote regularly of long-term and short-term goals and meticulously detailed how she organized her day — 20 minutes for breakfast, 20 minutes for jogging.

This photo provided by her family shows Yingying Zhang at the Summer Palace in Beijing. The 26-year-old visiting scholar at the University of Illinois at Urbana-Champaign, disappeared June 9, 2017. A former graduate student has been charged with kidnapping and killing her. Zhang's body has not been found. (Family Photo via AP)

On June 1, her last entry, Zhang was a bit more philosophical.

"Life," she wrote, "is too short to be ordinary."

Hou said it will be hard to leave and wait from thousands of miles away to bring Zhang home.

"We don't know how much longer this journey is going to be," he said. "We just feel hopeless."

When you unlock a phone, step into view of a security camera or drive past a license plate reader at night, beams of infrared light - invisible to the naked eye — shine onto the unique contours of your face, your body, your license plate lettering. Those infrared beams allow cameras to pick out and recognize individual human beings.

Over the past decade, facial recognition technology has gone from science fiction fantasy to worldwide reality — nowhere more so than in China, home to more security cameras than the rest of the world combined.

At airports and train stations, passengers line up for face scans at gates and by officers.

On the streets, cameras scan pedestrians and flag vehicles breaking traffic rules.

By law, anyone registering new SIM cards in China must show themselves to a face scanning camera, the images stored in telecom databases. And until recently, Chinese authorities required most guests to scan their faces when checking in to a hotel.

For many, such technology has offered convenience and safety, seamlessly woven into the backdrop of their lives. But for some, it's become an intrusive form of state control.

Associated Press investigations have found that such surveillance systems in China were to a large degree designed and built by American companies, playing a far greater role in enabling human rights abuses than previously known. It has cemented the rule of the Chinese Communist Party, offering it a powerful tool to control and monitor perceived threats to the state like dissidents, ethnic minorities and even its own officials.

Dozens who spoke to AP, from Tibetan activists to ordinary farmers to a former vice mayor, described being tracked and monitored by vast networks of cameras that stud the country, hampering their movements and alerting the police to their activities.

For years, such technology faced legal barriers in the country where it was first developed, the United States. But over the past five years, the U.S. Border Patrol has vastly expanded its surveillance powers, monitoring millions of American drivers nationwide in a secretive program to identify and detain people whose travel patterns it deems suspicious, AP found.

Under the Trump administration, billions are now being poured into a vast array of surveillance systems, including license plate readers across the U.S. that have ensnared innocent drivers for little more than taking a quick trip to areas near the border.

In this series of photographs, an infrared filter was used on a modified camera converted to capture the full spectrum of light, including ultraviolet, visible, and infrared.

This filter, which cuts out some visible light to better reveal infrared, is red by design in order to block certain light wavelengths.

AP photographers on three continents snapped photos showing how these beams are used to track vehicles and people, enable facial recognition - and ultimately, assert digital control.

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This is a documentary photo story curated by AP photo editors.

Retired Chinese official Li Chuanliang, who openly criticized the Chinese government after seeing first-hand how surveillance technology built up the government's power and is now being accused of corruption by Beijing, is illuminated by cellphone infrared facial recognition beams as he stands for a photo in the oil fields of Midland, Texas, where he's currently living in exile, Oct. 11, 2025. (AP Photo/David Goldman)

An infrared beam of light shines from a security camera watching over the Wudaoying alley in Beijing as pedestrians walk by, Oct. 10, 2025. (AP Photo/Ng Han Guan)

Pastor Pan Yongguang, right, and his son Paul, members of a Chinese church living in exile after fleeing from China, are illuminated by cellphone infrared facial recognition beams while sitting for a photo in the community room of the ranch compound where they're living in Midland, Texas, Oct. 11, 2025. (AP Photo/David Goldman)

Alek Schott drives past the infrared beam of an automatic license plate reader recording vehicles along Interstate 10, a route he occasionally takes for work trips, near Seguin, Texas, Oct. 15, 2025. (AP Photo/David Goldman)

An infrared beam of light shines out of an automatic license plate reader recording vehicles passing along U.S. Highway 83, Oct. 13, 2025, in Laredo, Texas. (AP Photo/David Goldman)

Dong e Li, a member of a Chinese church living in exile after fleeing from China, is illuminated by cellphone infrared facial recognition beams as she sits for a photo in her home, Oct. 13, 2025, in Midland, Texas. (AP Photo/David Goldman)

An infrared beam of light shines from a security camera watching over the Wudaoying alley in Beijing, Oct. 10, 2025. (AP Photo/Ng Han Guan)

Former Xinjiang government engineer Nureli Abliz, who saw firsthand how surveillance technology flagged thousands of people in China for detention, even when they had committed no crime, is illuminated by cellphone infrared facial recognition beams as he sits for a photo in Mannheim, Germany, where he's currently living in exile, July 23, 2025. (AP Photo/David Goldman)

Guigiu Chen, who escaped China with her daughters after her husband, prominent rights lawyer Xie Yang, was detained, is illuminated by cellphone infrared facial recognition beams as she sits for a photo, Oct. 12, 2025, in Midland, Texas, where she's living in exile. (AP Photo/David Goldman)

Infrared facial recognition beams are emitted from a cellphone held by a Uyghur man, who asked to remain anonymous for safety reasons, as he’s photographed in front of the U.S. Capitol, Oct. 16, 2025, in Washington, where he’s living in exile after escaping China. (AP Photo/David Goldman)

An infrared beam of light shines out of an automatic license plate reader as it records vehicles driving by, Oct. 13, 2025, in Laredo, Texas. (AP Photo/David Goldman)

An image of the Dalai Lama and Namkyi, a Tibetan former political prisoner who was arrested and imprisoned at 15 for protesting Chinese rule, are illuminated by cellphone infrared facial recognition beams as Namkyi sits for a photo at the Office of Tibet, Oct. 7, 2025, in Washington. (AP Photo/David Goldman)

Alek Schott, who filed a lawsuit alleging violations of his constitutional rights when Texas sheriff's deputies stopped and searched his vehicle at the request of Border Patrol agents, is photographed, as an infrared beam of light from a Flock Safety automatic license plate reader records passing vehicles driving near his neighborhood, Oct. 16, 2025, in Houston. (AP Photo/David Goldman)

Wensheng Wen, rear right, and his wife, Lou Guangyzing, along with their children Xin, 11, from right, Gehua, 9, Jinghua, 3, Rou, 6, and Younghua, 3, members of a Chinese church living in exile after fleeing from China, are illuminated by beams of pulsed laser light from a cellphone's LiDAR scanner as they sit for a photo, Oct. 12, 2025, in Midland, Texas. (AP Photo/David Goldman)

An infrared beam of light shines out of an automatic license plate reader recording vehicles passing along U.S. Highway 83, Oct. 13, 2025, in Laredo, Texas. (AP Photo/David Goldman)

Yang Guoliang, who has been under surveillance by Chinese officials after complaining about a land dispute to the central government in Beijing, is illuminated by cellphone infrared facial recognition beams as he smokes a cigarette inside his home in Changzhou in eastern China's Jiangsu Province, Sept. 7, 2025. (AP Photo/Ng Han Guan)

An infrared beam of light shines from a security camera watching over the Beigulou alleyway in Beijing as a pedestrian passes, Oct. 9, 2025. (AP Photo/Ng Han Guan)

Former Xinjiang government engineer Nureli Abliz, who saw firsthand how surveillance technology flagged thousands of people in China for detention, even when they had committed no crime, is illuminated by cellphone infrared facial recognition beams as he sits for a photo in Mannheim, Germany, where he is currently living in exile, July 23, 2025. (AP Photo/David Goldman)