Fire ravaged the upper floors of a six-story office building, killing three people and injuring at least six others as firefighters using ladder trucks helped survivors escape through broken windows.

Dawn Herring was in a fourth floor office when the fire started and said she never heard an alarm.

"We didn't realize there was a fire going on until I heard somebody scream," Herring told a cable news network Thursday. When she and her colleagues tried to leave, they found the hallways and both stairways filled with smoke.

"We had no other choice but to go back into the office," Herring said. "We finally broke a window and we waited and waited. It seemed like forever for the fire department to bring the ladder over to our window.

"It's a wake-up call. It makes you realize how thankful you really are to be alive."

Authorities believe the fire broke out in a medical supply firm on the fifth floor, but they were still investigating the cause.

Flames began shooting from the building's top two floors late Wednesday afternoon just as the business day was wrapping up. Heavy smoke blanketed a nearby 10-lane freeway during the evening rush hour.

Roy Anderson and Larry Gill, who work at Rail Crew Express on the sixth floor, said they were outside when they heard an explosion and glass shattering.

As the fire spread, people still in the building called 911 for help, saying they were trapped in offices that were filling with smoke, District Fire Chief T.J. Dowdy said. He said firefighters eventually found three bodies on the building's fifth floor, two in the same office. The victims and their companies weren't immediately identified.

Four other people were injured and taken to Memorial Hermann-Texas Medical Center, where a spokesman said one was in critical condition and the others were in fair or good condition.

J Systems owner Jim Jimenez, whose office is on the fourth floor, said he noticed the smoke in the atrium and quickly got out of the building. As he looked back, he saw the flames.

"It looked like the entire suite was on fire," Jimenez said. "It just took seconds."

The building, constructed of glass and masonry in the early 1980s, sits on the 610 Loop, a busy highway. An engineering firm and several medical clinics are listed as tenants.

Herring said that in the 18 months she had worked there, she had never been involved in a fire drill or heard the fire alarm, and she said she never saw sprinklers go off.

Boxer Property Management Corp., which manages the building, declined to comment.

Workers in a Chicago high-rise office tower also got a scare Wednesday when fire broke out in equipment on their roof. The blaze on the 45-story downtown building didn't spread beyond the roof, officials said, and firefighters had it out in about two hours.

Another high-rise fire late Wednesday in California destroyed an apartment in an oceanfront tower in Long Beach. One resident jumped to his death from an 18th-story balcony, said firefighter Will Nash, a spokesman for the Long Beach Fire Department.

"There was heavy amount of fire coming out of his apartment and he had no where else to go," Nash said.