BRISTOL, Pa. (AP) — A thunderous explosion at a nursing home just outside Philadelphia killed at least two people, collapsed part of the building, sent flames shooting out and left people trapped inside, authorities said.

Pennsylvania Gov. Josh Shapiro said in a news conference several hours after the explosion that at least two had been killed.

The explosion happened at Bristol Health & Rehab Center in Bristol Township, just as a utility crew had been on site looking for a gas leak, although the cause of the explosion was unclear several hours later, as was 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.

Bucks County emergency management officials said they received the report of an explosion at approximately 2:17 p.m. and said a portion of the building was reported to have collapsed. Ruth Miller, a Pennsylvania Emergency Management Agency spokesperson, said her agency had been informed that people were trapped inside.

Willie Tye, who lives about a block away, said he was sitting at home watching a basketball game on TV when he heard a “loud kaboom.”

“I thought an airplane or something came and fell on my house,” Tye said.

He got up to go look and saw “fire everywhere” and people escaping the building. The explosion looked like it happened in the kitchen area of the nursing home, he said. Tye said some of the people who live or work there didn’t make it out.

“Just got to keep praying for them,” Tye said.

The cause of the explosion was unclear.