CHICAGO – The burning building had been vacant for years, but the firefighters went in anyway — just in case squatters started the blaze and were trapped inside. Then the heavy-timbered roof and a wall collapsed.

Four firefighters were trapped under debris, and two of them died on a day that already was among the most somber on the Chicago Fire Department’s calendar. Exactly 100 years ago, 21 Chicago firefighters died when a wall collapsed at the Union Stock Yards fire, one of the nation’s worst tragedies for firefighters before 9/11.

“We were ringing the bell and calling out the names,” said retired fireman Bill Cosgrove, who was at a service honoring the anniversary. “We heard a mayday on the radio that a wall had fallen in.”

Most of the firefighters at the service broke down in tears when they found out about the collapse, he said.

“It was beyond disbelief,” Cosgrove said. “It was a matter of a few hours and a hundred years later we have the same type of incident.”

He said two firefighters at the memorial left to help dig out their colleagues. Other off-duty firefighters rushed there as well, said fire department spokesman Larry Langford.

They joined more than 170 other firefighters on duty who responded to a 911 call about the burning building just before dawn, Langford said.

He said no one expected the call to be anything more than a routine winter fire.

Although the one-story building had been vacant for years and the utilities had been turned off, firefighters searched the building out of concern that homeless people might be inside trying to stay warm.

The cause of the fire was under investigation, Authorities speculated that squatters might have been burning debris to keep warm.

“The fire had no other way of starting,” Langford said.

He said the only people injured were firefighters.

The men killed were Edward Stringer, 47, a 12-year department veteran, and Corey Ankum, 34, who joined the department a little over a year ago. They and two others were trapped under the roof debris.

Two firefighters were pulled out quickly but rescuers had to use extrication equipment to reach the other two.

All firefighters “did the best they could to save their brothers,” said Robert Hoff, the city’s fire commissioner.