Chicago ? The gunman who went on a deadly shooting spree in a downtown high-rise law office went to the building in search of an attorney because he felt cheated over an invention, authorities said Saturday.

Joe Jackson, 59, made at least one other attempt Friday to enter the offices of the intellectual property law firm Wood, Phillips, Katz, Clark & Mortimer, but was turned away because he didn’t have an appointment, said Chicago Police Superintendent Phil Cline.

The next time he returned, Jackson had a revolver, knife and hammer hidden in a manila envelope, Cline said. He forced a security guard at gunpoint to take him up to the 38th floor, where shooting victim Michael McKenna, 58, rented office space. Jackson carried McKenna’s business card in his pocket, Cline said.

Then Jackson chained the doors behind him, grabbed a hostage and started shooting, as he ranted to witnesses that he had been deceived over his invention, a toilet for a truck, Cline said.

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

He was holding a hostage at gunpoint when two SWAT officers shot him in the face and chest from about 45 yards away, Cline said. 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.

Jackson died of multiple gunshot wounds, according to the medical examiner’s office, which identified the other victims as attorney Allen J. Hoover, 65, of Wilmette, and Paul Goodson, 78, of Chicago, a retired school teacher.

Colleagues said 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.

A fourth victim, Ruth Zak Leib, 57, of Oak Park, was identified by police as McKenna’s longtime paralegal. She was treated at a hospital and released Friday for a gunshot wound to the foot, a hospital spokeswoman said.

Wood Phillips said in a statement Saturday it would be closed Monday “as all employees gather together off site to support each other, to remember our colleagues and to meet with grief counselors.”

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.

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.

Authorities did not fault the security guard for taking Jackson to the 38th floor.

“This guy’s telling him, ‘Take me upstairs. Don’t say anything,'” Cline said. “He followed instructions.”

About 45 minutes elapsed between the first 911 calls and the SWAT team shooting, police said. Chicago Mayor Richard Daley said police did a “tremendous” job.

Cline described a blood-spattered crime scene covered by broken glass. He praised paramedics and firefighters.

“It was a very chaotic scene. We had one of the individuals who was shot to death laying in the hallway. We had other individuals who were shot and we got them out right away,” Cline said.