The firefighters who died have been identified as Firefighters Edward Stringer, 47, and Corey Ankum, 38. The two were helping sweep a vacant building after a 3-11 fire when the roof and a wall collapsed.
Stringer, Ankum and two other firefighters became trapped under the debris.
A "mayday" was called and hundreds of firefighters were sent to the scene in the rescue. More than a dozen others were injured, but they all are expected to be OK.
Despite a frantic rescue effort, Stringer and Ankum died of their injuries.
Friday night, an emotional Mayor Richard Daley urged everyone to pray for the families of Stringer and Ankum, as well as the 17 other firefighters who were injured.
"Today's tragic fire demonstrates the selflessness and commitment of the dedicated men and women who work 24 hours a day, 7 days a week to ensure the safety of all of our citizens," said Daley at a news conference.
Chicago Fire Commissioner Robert Hoff echoed Daley's concerns for the affected families.
Our major concern right now... is their families," said Hoff at the same news conference. "Their families are number one on our list right now."
Stringer was a 12-year veteran of the Chicago Fire Department. Ankum had been with the department for just 18 months. Before that, he was a police officer.
"Those who were killed, those who were injured, and everyone on the scene knows they were doing a job that was very dangerous, but they also know that job is very important and is essential to our city, to our neighborhoods and our homes," said Chicago fire union chief Tom Ryan. "No matter how much experience you have on the job, a morning like this still takes you by surprise."
Stringer was the father of two adult children, and Ankum was the husband of a longtime executive assistant to Mayor Daley.
"His wife Tamika has been one of my closest assistants - a true friend and a confidante with me," said Daley, his voice breaking repeatedly with emotion. "Our condolences and prayers go out to both families."
All of the other firefighters injured are expected to recover. Five firefighters remained hospitalized Wednesday night in good or stable condition. The others were not seriously hurt, according to the Chicago Fire Department, and have been treated for minor injuries.
Frantic search for trapped firefighters
Officials said as many as six firefighters were going over hot spots after a 3-11 fire and searching the vacant building on East 75th Street near Stony Island for homeless people when part of the building gave way and the roof came down on four of them.
"They went up on the ladder, ladder went on down. Next thing you know, they must (have) got onto the top part of the roof, and the roof just caved in," witness Kimberly Lofton said.
"They were below. They were inside the structure," Chicago Fire Commissioner Robert Hoff said. "The search effort was aggressive. We had alarms going off. Two members were found immediately. A couple of members, to get at we had to use extrication devices-- but we located them immediately."
A "mayday" was called in and a response team, the Rapid Intervention Team (RIT), was sent. More than 60 Chicago Fire Department companies responded and conducted a frantic search for colleagues.
"The RIT company went to work right away. They removed, I couldn't tell you each and every spot, but firefighters were removed from different corners of the building, and they worked hard and they got them out fast," Commissioner Hoff said.
The deadly collapse came 100 years to the day after 21 firefighters died in a fire at the Union Stock Yards.
All firefighters were accounted for at 8:50 a.m., but crews continued to search the rubble to make sure no homeless people were inside the building at the time of the fire or collapse.
"People in this weather take refuge," said Commissioner Hoff. "We take no building as being vacant."
"Over the years, we have had numerous squatters come in and try to use the place as a home and so forth. We have called the police department numerous times. They have came out and made sure the place was secured, but unfortunately, squatters still come through and break in and try to have refuge inside the place," said Joerico Smart, who owns a carwash nearby.
City: Building had code violations
The cause of the fire and the collapse remain under investigation- but officials will be looking closely at code violations at the building at 1744 E 75th Street.
The building was once occupied by Banner Dry Cleaner, but it was not clear Wednesday how long it had been vacant.
The building owner had been cited by the city for numerous building code violations-- including the roof. Inspectors wrote that the owner had failed to maintain the roof in sound condition and repair, that there were holes in the roof with leaks, and that the trusses of the leak were vented and rotted.
Officials say the roof and its trusses never caught fire and that they do not believe the collapse was fire-related. Documents indicate that the building had not been maintained properly. The cause of the fire and collapse are still under investigation.
A consent decree was reached between that owner and the city and repairs were to have been made last month or the building was to be sold.
STATEMENT OF MAYOR RICHARD M. DALEY ON THE DEATHS OF FIREFIGHTERS EDWARD STRINGER AND COREY ANKUM
On behalf of all the residents of Chicago, I extend my condolences to the families of Firefighters Edward Stringer and Corey Ankum, who died today in the line of duty. I knew Cory Ankum and his family and I share in their loss today.
Our prayers go out to the families of these two firefighters and to their brothers and sisters in the Chicago Fire Department, who put their lives in danger every day to keep Chicagoans safe.
And our thoughts and prayers are with the other brave firefighters who remain hospitalized as a result of today's fire.
The deaths of Firefighters Stringer and Ankum are both a sad reminder of how much gratitude we owe our first responders and a tragedy for all Chicagoans.