New Delhi, Dec. 24 -- A thunderous explosion at a nursing home just outside Philadelphia collapsed part of the building and left people injured and possibly trapped inside, authorities said.

The explosion happened at Bristol Health & Rehab Center (Silver Lake Healthcare Center) in Bristol Township, just as a utility crew had been inside looking for a gas leak, although the cause of the explosion was unclear several hours later, as were the extent of the casualties.

A plume of black smoke rose from the nursing home, as emergency responders, fire trucks and ambulances from across the region rushed there, joined by earthmoving equipment.

Authorities said there were injuries, but had yet to say whether there were any fatalities.

Police Lt...