A man living in an underground bunker along the Milwaukee River had been there for a “considerable period” with a stash of weapons that included a bow with arrows fashioned from “snowplow stakes,” a Wisconsin sheriff said Friday.

Milwaukee County Sheriff Earnell Lucas said it’s possible that 41-year-old Geoffrey Graff had been living in the bunker for “maybe years” but that authorities don’t know for certain. Graff came to authorities’ attention when residents reported shots fired Wednesday morning near a wooded area overlooking the river.

Upon arriving, sheriff’s deputies and officers from three police departments discovered a concealed entrance to an underground encampment that was 8 feet (2.4 meters) deep, 8 feet (2.4 meters) wide, and 20 feet (6.1 meters) long, Lucas said. WTMJ-TV video shows debris over a large area in the woods and a bunker with a cloth partially covering the opening.

“One would wonder how in an urban setting an individual is able to hide in plain sight,” Lucas said.

He said when officers found the bunker Graff immediately incriminated himself, telling them he had fired three shots into the river because he was angry at his dogs for running away.

Inside the bunker, authorities found two shotguns, a pistol, a long rifle, and the bow with snowplow stakes for arrows, the sheriff said. The bunker also had a grill, propane tanks, a generator and various power tools, Lucas said.

Milwaukee prosecutors are considering charges that could include recklessly endangering public safety, trespassing and carrying concealed weapons. It’s not known whether Graff has a lawyer.

“Let’s just be thankful that this didn’t manifest itself in a way, that it could’ve been more. We could be investigating something totally different,” Lucas said.

Investigators haven’t found any ideology or political motive behind Graff’s alleged actions, said the sheriff, who emphasized there is no danger to the public.