Carrying a cache of weapons into a bustling downtown office, a man chained a law firm's doors closed and fatally shot three people before a police sniper killed him as he held a hostage at gunpoint, authorities said.

The standoff at the 43-story Citigroup Center, which also houses a train station, sent office workers fleeing and stranded commuters Friday as the work week drew to a close.

Police entered through another door in the U-shaped law office, and a SWAT officer shot the gunman from about 45 yards away, Police Superintendent Phil Cline said.

There were no negotiations and the hostage was unharmed, police said.

Cline said the gunman carried a revolver, knife and hammer in a large manila envelope and had security escort him to the Wood, Phillips, Katz, Clark & Mortimer office on the 38th floor.

The gunman, who was escorted to the office by building security, didn't work in the office but demanded to see one of the victims, police said.

"He was not employed there, but we feel he did have previous encounters with the individuals in that office," Cline said.

The Cook County Medical Examiner's office identified three of the victims early Saturday as Michael R. McKenna, 58, of Chicago; Allen J. Hoover, 65, of Wilmette; and Paul Goodson, 78, of Chicago.

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

The medical examiner's office had not identified the gunman early Saturday but said he was in his 60s.

Police identified the fourth victim, Ruth Zak Leib, 57, of Oak Park, as McKenna's longtime paralegal. She was taken to the hospital with a gunshot wound to the foot, but was released Friday night, Rush University Medical Center spokeswoman Kim Waterman said.

Wood Phillips specializes in patent and intellectual property law, according to its Web site.

A partner at the firm, Stephen D. Geimer, declined comment Friday night.

Fire officials said they received reports of shots fired on the 38th floor around 3:15 p.m. There were about 30 people on the floor, they said.

The shooter "grabbed a hostage and he was pointing a gun alternately at the hostage's head and his own head," Cline said.

Cindy Penzick, secretary in a law firm on the 37th floor, said that after a co-worker told her she heard gunshots, a police officer with his gun drawn on their floor yelled at them to get out.

Penzick said she is usually calm, "but I have to tell you this was scary as hell."

People hurried down the escalators and ran from the skyscraper on the west side of downtown.

Keegan Greene, who works at Verizon Wireless on the first floor, was helping a customer when fire alarms began going off.

"One of the security guards came up to us and started saying, "Run, run, run, run, run!" Greene said.

Service on the Metra commuter line was suspended for more than an hour and area buses were diverted while the incident unfolded. Metra spokeswoman Judy Pardonnet said all Metra crew members had been locked into secure areas before train service resumed just after 5 p.m.

Janet Buswell, an office manager on the 25th floor, said her staff learned of the incident when emergency flashers went off and they were told over building speakers to secure their offices and let no one in or out.

"It was a little tense, when you don't know what's going on," Buswell said.