A stubborn fire raged for five and a half hours on the 29th floor of a historic downtown skyscraper, sending at least 34 people to area hospitals, most of them firefighters who suffered smoke inhalation. Some injuries were serious, but no deaths were reported.

Streets around the Art Deco-style (search) building, which serves as the corporate headquarters of LaSalle Bank (search), were closed early Tuesday as city officials were trying to determine its structural soundness. The cause was still unknown.

More than 300 firefighters battled the blaze Monday night that sent flames belching from the building's upper-story windows. Firefighters shot water into the windows, sometimes standing on the building's wedding cake-like tiers to gain better access.

Of the 34 people injured, 22 were firefighters in moderate to serious condition, said fire commissioner Cortez Trotter. Most were being treated for smoke inhalation or minor injuries, officials said.

For firefighters, it was their first major ordeal since a fatal high-rise fire in a county office building 14 months ago that killed six people. Office workers who escaped Monday's blaze in the 43-story building said firefighters escorted them through blinding smoke to safety.

Bob Bailey, a partner in a commercial real estate law firm on the building's 39th floor, said he had to keep his head outside a window or near the ground because of the smoke until firefighters came and led him down an elevator.

"We had our windows open in the office and I had to put my coat on the door, so that smoke wouldn't start rolling in," he said. "And for a while, we weren't sure we were going to make it."

Trotter said firefighters had searched all the building's stairwells and floors they could gain access to early Tuesday morning, but that some hotspots remained in the building.

A spokesman for the Cook County medical examiner's office said it had not been notified of any deaths.

The fire at 135 S. LaSalle Street was reported about 6:30 p.m. and extinguished about midnight. Thick black smoke poured out of windows, and metal window frames were twisted by the heat of the blaze on the 29th and 30th floors.

More than one-third of the city's fire equipment was at the scene, and suburban fire departments sent crews into the city to act as backup.

Jim Rubens, who works at a law firm in the building, said he held hands with other victims as firefighters escorted them down a smoky stairwell.

"It was horribly thick smoke and the halls were completely dark," said Rubens, who was sweating and covered in black soot. "And we were trying to touch the person in front of you to see where we were going to."

Tom Smith, a lawyer who worked in the building, said a firefighter escorted him to a freight elevator after smoke in a stairway turned him back.

Sarah Nadelhoffer, a lawyer who worked on the 39th floor, said she was working late when her office started to fill up with smoke.

She and co-workers were forced into another office, where they opened a window to get fresh air. They stuffed a coat under the door to block the smoke, which was getting thicker.

"I was thinking it can't be over this way," she said. "I also thought I have no control. I'm going to pray the fire department gets me out."

The fire comes 14 months after a 35-story building in downtown owned by Cook County caught fire, killing six people. A state-funded investigation of the October 2003 blaze concluded in September that the deaths could have been prevented if there had been sprinklers and unlocked stairwells, and if firefighters had searched for victims sooner and kept out smoke and heat.

The state report also cited inadequate evacuation training of building staff and occupants, and poor communication among fire and police emergency dispatchers as well as the city's emergency dispatchers and fire commanders at the scene.

LaSalle Bank spokesman Shawn Platt said about 3,000 people work in the building. Most people there work normal business hours, but some departments are open 24 hours, he said. The 29th floor holds the bank's trust division, he said.

Trotter said there were 400 to 500 people in the building at the time of the fire.

The bank conducted a safety drill about a month ago, and Platt initially said the building had sprinklers, although he did not know on which floors they were located. Later Monday night, Platt said the building was putting in the infrastructure for sprinklers and that there weren't any on the 29th floor.

Chicago-based Lasalle Bank has $65.1 billion in assets and $35 billion in deposits and is one of the largest banks in the Midwest. Its building was originally named the Field Building, after Chicago retailer Marshall Field, whose estate developed the skyscraper in the early 1930s. It was designated a Chicago landmark in 1994.