The aftermath of a pair of violent explosions at a nursing home in Bucks County, Pennsylvania came into clearer view Wednesday as federal investigators begin their examination of the blast and fire that killed a staff member and resident and injured at least 20 others.

A collapsed roof, blown out windows and charred rubble were all visible from CBS Philadelphia's Chopper 3, which flew over the Silver Lake Nursing Home in Bristol Township Wednesday morning.

Officials said they believe a gas leak led to two explosions and a fire just after 2 p.m. on Tuesday, Dec. 23. In a statement, PECO said crews were responding to a gas odor report at the building a few hours before the explosion, and turned off the natural gas and electric service to the facility.

"It is not known at this time if PECO's equipment, or natural gas, was involved in this incident," PECO said Tuesday.

The National Transportation Safety Board is now leading the investigation.

In one section of the building, video shows that the roof is completely crumbled. Officials said Tuesday that part of the first floor of the facility collapsed into the basement.

One neighbor said the explosion "sounded like a bomb."

"I'm sad, heartbroken," said Gertrude Montgomery, whose backyard had a clear view of the collapse site. "These people here, you're in your bed, you're thinking you're fine, you're in a nursing home, you're safe, and this happens."

The two victims killed have not yet been publicly identified, and it's unknown how many residents and staff were inside the building at the time of the explosions.