BRISTOL — A thunderous explosion Tuesday at a nursing home just outside Philadelphia killed at least two people, collapsed part of the building, sent fire 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 after emergency responders braved the flames, a heavy odor of gas and a second explosion to evacuate residents and employees.

Fire officials said they were in “rescue mode” five hours later, with responders still digging by hand and using search dogs, earth-moving equipment and sonar to locate potential victims.

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.

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 did not identify those who died and did not know the total number of those injured after residents and employees were evacuated to a number of different hospitals.

Shapiro asked his fellow Pennsylvanians to take a moment to pray “for this community, for those who are still missing, for those who are injured, and for those families who are about to celebrate Christmas with an empty chair at their table.”

The town’s fire chief, Kevin Dippolito, said at the Tuesday evening news conference that there were five people still unaccounted for, but he cautioned that some may have left the scene with family members.

Dippolito described a chaotic rescue where firefighters found people stuck in stairwells and elevator shafts, and pulled residents out of the fiery building through windows and doors. Two people were rescued from a collapsed section of building, he said.

Firefighters handed off patients to waiting police officers outside, including one “who literally threw two people over his shoulders,” Dippolito said. “It was nothing short of extraordinary.” A second explosion erupted during the rescue, he said.

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.

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.

Shapiro said a finding that the gas leak caused the explosion was preliminary.

The local gas utility, PECO, said its crews had responded to reports of a gas odor at the nursing home shortly after 2 p.m.

“While crews were on site, an explosion occurred at the facility. PECO crews shut off natural gas and electric service to the facility to ensure the safety of first responders and local residents,” the utility said in a statement.

The 174-bed nursing home is about 20 miles northeast of Philadelphia. It is affiliated with Saber Healthcare Group, which said it was working with local emergency authorities.