CHICAGO (AP) - A gunman in a downtown skyscraper shot four people, killing three of them, and took a hostage before being killed after police fired at him, authorities said.

The shootings Friday in the top floor of the 38-story Citigroup Center, which also houses a train station, sent office workers fleeing and stranded rush-hour commuters.

Authorities said earlier that three people were hospitalized in critical condition and one was in stable condition; besides the gunman, it was unclear who died.

The gunman, armed with a snub-nose revolver, knife and hammer, shot the four victims over ''several minutes,'' police Superintendent Phil Cline said.

He was holding the hostage when police shot at him, but ''before the offender died, he may have also shot himself,'' Cline said. Police said they will not know for sure until an autopsy is performed.

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. Dozens of police cars and two fire trucks were in front of the building, where one person was seen being loaded onto an ambulance.

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.

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

AP-ES-12-08-06 2041EST