Omar Thornton, 34, walked into a room early Tuesday at the beverage distributorship where he worked in Manchester, Connecticut.
Company and union officials played a video. He calmly watched images that purportedly showed him stealing from a truck.
An employee of Hartford Distributors for two years and a driver for one, Thornton was given a choice of resigning or being fired, union and company officials said.
He signed a resignation paper, was escorted out of the room and toward the door. He asked for a drink of water, company CEO Ross Hollander told CNN. Then, Thornton pulled out a handgun and began firing, officials said.
Within minutes, the gunman and eight other people were dead, and two people were wounded, according to Manchester police. Police swarmed to the scene, but never had a chance to talk with Thornton, who apparently shot himself to death in an office.