CHICAGO - The burning building had been vacant for years, but the firefighters went in anyway - just in case squatters 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.

"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."

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 9-1-1 call about the burning building just before dawn yesterday, Langford said.

Although the one-story building, which was more than 50 years old, had been vacant for years and the utilities had been turned off, firefighters searched it 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 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.