Authorities are investigating an explosion and fire Monday morning at a house in Butler.

The fire at about 1 a.m. left the two-story home at 111 South St. a charred shell — but no one was injured.

“The explosion was bad enough that it blew out the front door,” said Capt. Ted Codispot of the Butler Fire Department. “Two pieces of the door were blown all the way across the street.”

City police evacuated neighbors, including two families that lived next door on opposite sides of the house.

The burning house was vacant. Butler police said the most recent occupants were not home — they were in the Butler County Prison.

The pair was arrested four days earlier on drug charges, police said.

The state police fire marshal's office is investigating but a definitive cause may never be known due to the extensive damage.

“The fire marshal was on scene about 4 a.m. and his initial inspection revealed it may be related to the gas furnace,” said Butler police Deputy Chief David Adam.

Trooper Luke Nelson, deputy fire marshal, said today that his investigation into the cause and origin of the explosion and fire is complete, but didn't reveal much. Nelson said the cause of the explosion remains undetermined.

“There's just too much damage and the structure itself (was) just too unstable to really get in and dig around in the area of interest, which would be the back of the house.”

Nelson said the incident has been classified as a “low order explosion,” meaning it was contained to a specific area of the home at 111 South St. and didn't cause damage to surrounding buildings.

He said that any further investigation into the case would be handled by Butler police.