A portion of an 85-year-old apartment building collapsed Monday morning, injuring seven people, some seriously, fire officials said.

Witnesses reported an explosion before the collapse on the city's West Side.

"We believe it was a blast of some kind, but we don't know what fueled it," Fire Department spokesman Larry Langford said.

He said one side of the horseshoe-shaped brick building collapsed to the ground, but that portion of the structure was not occupied and covered the sidewalk and street, he said.

All of the tenants were accounted-for later Monday morning after fire crews searched for victims, Langford said. After the search wound up, a demolition crew began clearing the debris.

A few of the seven people injured were taken to hospitals in serious condition, but none of the injuries initially appeared to be life-threatening, Langford said.

The 85-year-old building had 24 units, but only 15 people lived there, said Pete Scales, a spokesman for the city's Department of Buildings.

The American Red Cross (search) planned to provide short-term living accommodations for the displaced tenants, spokeswoman Cat Langel said.