Rapper Tekashi 6ix9ine (six-NYN') is testifying against two reputed members of a violent New York City street gang.
The rapper took the witness stand Tuesday as part of an agreement with federal prosecutors.
He admitted in a guilty plea this year that he joined the Nine Trey Gangsta Bloods and helped try to kill a rival gang member.
One of the defendants, Anthony Ellison, is accused of kidnapping and robbing 6ix9ine last year after a falling-out. Ellison denies the charges.
6ix9ine's decision to cooperate has upended his bad boy image on social media. Rap icon Snoop Dogg recently labeled him a "snitch."
The 23-year-old rapper's album "Day69: Graduation Day" was among the top sellers on iTunes following its release last year.
This story has been updated to correct the spelling of the rapper's name in the headline to 6ix9ine, instead of 6six9ine.