PLUM, Allegheny County — Investigators said Friday the location of an explosion that killed six people and destroyed three houses near Pittsburgh last month was inside one of the homes, ruling out an outside cause.

The Allegheny County fire marshal's office said all potential outside factors were eliminated, including wells, pipelines and other utilities, pinpointing the source as a home on Rustic Ridge Drive in Plum Borough.

The cause of the explosion remains under investigation, the office said, and there is no time frame for when the probe will be completed. The investigation at the scene of the explosion has been completed and the property is being returned to its owners, the agency said in a release.

That house and two others were so badly damaged they must be demolished, and 10 others were not safe for residents in the weeks after the Aug. 12 blast about 20 miles east of Pittsburgh.

All of the victims were found at the home. Officials have said one was found outside a garage, four were in the basement and the sixth was located and rescued from the rear yard. He was taken to a trauma center where he died several days later.

State officials had previously ruled out natural gas from an abandoned underground mine as a cause. Authorities have also said the home's owners had been having problems with their hot water tank.