Authorities had distributed the 20-year-old man's photo to Capitol police after learning about the online threat, Gov. Scott Walker's administration said in a statement. Police spotted him as he entered the Capitol, where he allegedly told officers he had a molotov cocktail in his backpack.
Police evacuated part of the building, and a bomb squad investigated the backpack outside. Authorities haven't said what was inside the backpack, though no injuries were reported.
Charges are pending, the statement said. Walker's spokesman declined comment.
It's unclear whether the man had political motivations. Police said his Facebook threat wasn't made against a specific individual.
Federal court records show the man filed a short, handwritten lawsuit last month against several entities, including the University of Wisconsin football team and the Dane County sheriff. He said he wanted "my money 989 billion-teillion. ... Equitable distribution. I believe in retribution," but provided no explanation.
State court records show he was charged last month with resisting an officer. His sister had called police to report he wasn't in his right mind, and when police found him at a Madison mall, he told them he hit someone with his vehicle and then challenged the officers to a fight, according to court documents.
None of the court records indicate the man had an attorney.