Chicago (CNN) -- A fire that killed two Chicago firefighters and injured 17 earlier this week grew from wood or trash burning in the "rear portion of the building," the fire department said Friday.
An "open-flame ignition of ordinary combustibles," such as wood or rubbish, led to Wednesday's blaze, Chicago Fire Department spokesman Larry Langford said in a news release. There had not been any gas or electric service in the abandoned building -- located on the city's south side -- for months, he added.
The Chicago police department has opened a criminal investigation into the fire, Langford said.
On Wednesday, Fire Commissioner Robert Hoff said the building was boarded up in the front but the back was open, leading firefighters to think that homeless people may have sought shelter there.
The casualties came after a wall and roof of the commercial building, which is on East 75th Street, collapsed without warning.
Hoff told reporters Wednesday afternoon that none of the injuries was life-threatening. At that time, one firefighter was in stable condition, he said.
The commissioner identified those killed as Corey Ankum, who had been with the department less than two years, and 12-year veteran Edward Stringer. Both were inside the structure. Other firefighters were working the blaze from the roof.
CNN affiliate WGN aired footage of a line of firefighters saluting one body as it arrived at the medical examiner's office later Wednesday.
Chicago Mayor Richard M. Daley issued a statement saying, "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."
The deaths came on the 100th anniversary of the Chicago Stockyards fire, which killed 21 firefighters and the city fire marshal.
The fire was reported shortly before 7 a.m. in the office area of the building, Langford said.
The routine fire call became a multiple-alarm blaze after part of the roof collapsed as firefighters were inside, he said. Ankum, Stringer and two other firefighters were trapped by the collapse.
The commissioner said Wednesday that investigators will look at whether ice or weather was a factor, as well as the age of the building and its heavy roof.
"There are things that happen when buildings are aged, and that could have been one of the contributing factors in this case," Hoff said.
Langford said it was his understanding that both of the fallen firefighters had children. While the loss of a firefighter is always a tragedy, he said, "it's been compounded" by the deaths occurring so close to the holidays.
CNN's Sara Pratley contributed to this report.