CHICAGO (AP) – The gunman who fatally shot three people in a law firm’s high-rise office before he was killed by police felt cheated over an invention, authorities said Saturday.

Joe Jackson forced a security guard at gunpoint to take him up to the 38th floor offices of Wood, Phillips, Katz, Clark & Mortimer, which specialized in intellectual property and patents. He carried the revolver, a knife and hammer in a large manila envelope and chained the office doors behind him, police said.

Jackson, 59, told witnesses before he was shot that he had been cheated over a toilet he had invented for use in trucks, Police Superintendent Phil Cline said Saturday.

He was holding a hostage at gunpoint Friday when SWAT officers shot him from about 45 yards away, Cline said earlier. There were no negotiations and the hostage was unharmed, police said.

“He had already shot four people. He had reloaded his gun,” Cline said Saturday. There were 25 to 30 other people on the floor at the time, Cline said earlier.

Mayor Richard Daley said police did a “tremendous” job handling the situation.

The confrontation at the 43-story Citigroup Center sent office workers fleeing and stranded commuters who use a train station in the building.

The Cook County Medical Examiner’s office identified the victims Saturday as Michael R. McKenna, 58, of Chicago; Allen J. Hoover, 65, of Wilmette; and Paul Goodson, 78, of Chicago. The medical examiner’s office said Jackson died of of multiple gunshot wounds.

Colleagues told reporters Hoover was a partner at the firm and McKenna was a patent attorney who rented space from the firm and also had offices in suburban Northbrook and in Hawaii. They said Goodson worked part time at the firm, sorting mail and making deliveries.

Jackson had McKenna’s business card in his pocket, Cline said.

“We know he went there for Mr. McKenna, then he continued to shoot other people,” Cline said Saturday.

Cline said Jackson had tried at least one other time Friday to go up to the firm’s offices but was turned away.

Jackson had three criminal offenses on his record, Cline said. In 1968 he was arrested for unlawful possession of a weapon and in 1977 he was arrested for a stolen motor vehicle and disorderly conduct.

Police said McKenna’s longtime paralegal, Ruth Zak Leib, 57, of Oak Park, was wounded. She was treated for a gunshot wound to the foot and was released Friday night, Rush University Medical Center spokeswoman Kim Waterman said.

A message left for a partner at the law firm, Stephen D. Geimer, at his home on Saturday was not immediately returned, nor was an e-mail sent to another partner, Jeffrey Clark.

A call to a telephone listing for Jackson at an address provided by the medical examiner’s office reached a telephone company recording, which said the customer had requested the listing remain private.

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Associated Press writers Don Babwin, Deanna Bellandi, Dave Carpenter and Nathaniel Hernandez contributed to this report.

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