A concrete slab crashed through the roof Monday of a building under construction on the city's South Side, crushing two workers to death, authorities said.

A construction crane was trying to position the slab atop the four-story masonry building. The workers were on the second floor, Chicago Fire Department spokesman Larry Langford said.

"We don't know if it was crane failure or operator error or what at this point. We don't want to speculate," Langford said.

Emergency crews closed the construction site and were working to reinforce the floors with large jacks, Langford said.

The dead were identified as Jaime Jones, 33, and James Beemsterboer, 37