MIDDLETON, Wis. (AP) — The gunman in a Wisconsin workplace shooting was taken to a mental health hospital and barred from purchasing firearms in 2004 after telling South Dakota police that he thought co-workers were “talking bad about him,” according to court records and police.

Anthony Tong also said he believed his neighbors were spying on him after police were called to his Sioux Falls apartment. Officers handcuffed Tong for their own safety because he was acting delusional and paranoid, and seized a pistol and an AR-15 rifle from his home, according to South Dakota court documents.

The revelations raise questions about how Tong acquired the 9 mm pistol used in Wednesday’s attack at WTS Paradigm, a software company in Middleton, Wisconsin. Investigators allege the 43-year-old employee opened fire in his office, seriously wounding three people and grazing a fourth before officers killed him in a shoot-out.

Middleton Police Chief Chuck Foulke said during a news conference Friday that Tong had no criminal history, but said his run-in with police in South Dakota in 2004 resulted in a judge revoking his concealed-carry permit over mental health questions. That revocation would have red-flagged him on a gun background check, making it illegal for him to purchase firearms, Foulke said.

“It absolutely seems as though with some loophole he was able to get that firearm, and he should not have been able to do that,” Foulke said.

Foulke said a motive behind the attack remains unclear.

The Bureau of Alcohol, Tobacco, Firearms and Explosives was trying to track down the origin of the pistol used in Wednesday’s attack.