MADISON, Wis. — A man who shot and wounded four co-workers at a Wisconsin software company last week passed the firm's background check.

WTS Paradigm CEO Nate Herbst said Tuesday that 43-year-old Anthony Tong passed a background check when he was hired. The Wisconsin State Journal reports that Herbst said the company wasn't aware of Tong's mental health history and he didn't know Tong personally.

Tong, a WTS Paradigm employee, opened fire in the company's Middleton headquarters on Sept. 19, wounding four of his co-workers, three seriously. Police killed Tong in a shootout.

Court records indicate a South Dakota judge stripped Tong of his concealed carry permit in 2004 after Sioux Falls officers took him to a hospital's mental health unit on a 24-hour hold.