PLUM, Pa. -- One person was killed and another was seriously injured Wednesday afternoon in a house explosion at 171 Mardi Gras Drive in Plum Borough.

Two STAT MedEvac helicopters were called to the scene. A 4-year-old girl identified as Gianna Pettinato was blown out of the house and into the yard. She was flown to UPMC Mercy and then transferred to Children's Hospital in critical condition with fractured legs and facial burns.

Channel 11's Jodine Costanzo reported a female neighbor pulled the girl from the rubble and helped her until emergency officials arrived.

The girl's grandfather, Richard Leith, 64, was flown to UPMC Mercy with burns. He later died at the hospital.

Leith was watching Gianna at her parents' home. They have been identified as Nino and Tina Pettinato. Their 10-year-old daughter was in school at the time of the explosion.

Witnesses told Channel 11 there was some sort of explosion along Mardi Gras Drive shortly after 1:30 p.m. One neighbor said it sounded like a bomb went off. At least 15 houses suffered damage.

Resident Thomas Kelly said, "Our whole house shook. The house that blew up is completely gone."

Another resident told Channel 11 the immediate neighbors near the destroyed house do not have windows; the explosion blew them out.

Following the explosion, Channel 11 received numerous phone calls and e-mails to the station. One woman wrote, "I live about a thousand yards from the house that exploded. My house shook and the chandelier was swinging."

Dominion Peoples gas crews were called to the scene, including an investigator who will try and determine the cause of the explosion. A gas company spokesperson said no digging projects were slated in the area and there were no reports of a natural gas smell prior to the explosion.

All residents along Mardi Gras Drive were evacuated to Holiday Park Elementary School. The Red Cross is assisting evacuees.

Watch Channel 11 News and stay with WPXI.com for continuous updates.