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. Both are in the county prison on $100,000 bail on charges of felony drug possession.

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 Tuesday 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 and didn't cause damage to surrounding buildings. But the circumstances surrounding the arrest of the previous tenants, who are facing possession charges for methamphetamine, have left both neighbors and authorities openly questioning whether the explosion was tied to a drug manufacturing operation.

“Was something left unattended while they were gone? Did someone else come in? Anything's possible at this point,” said Nelson, adding that any further investigation into the case would be handled by Butler police.

Hours after the fire engines left Monday morning, a firefighter put “condemned” signs on the house.

“It's a safety issue,” Codispot said, “and we will try to have it taken down as soon as possible.”

Due to the Memorial Day holiday, the city building was closed and authorities were unable to immediately confirm the owner, who is believed to live in Florida.

The explosion rattled nearby homes.

“I heard one big boom and then glass shattering,” said Bruce Bryner, who lives in an upstairs apartment above a hair salon, just two buildings down.

He was in the living room watching a movie on TV when the unknown noise jarred him. “I didn't know what the h--- it was,” he said.

Bryner like other neighbors first wondered if what they heard was a car crash, possibly on nearby Center Avenue.

“I thought it was a wreck,” said Butch Scott, who was lying in bed watching TV at his home across the street only a few houses away.

Moments later, he heard voices. “People were yelling.”

The clamor outside awakened neighbor Stacy Gould, who slept through the initial explosion.

“I heard screams and sirens,” she said. “I looked outside and there were flames and smoke everywhere.”

“I was watching it from my window,” said neighbor James Randell, who heard people yelling “get out, get out.”

By the time he got outside for a closer look and to render aid, Bryner said, other neighbors were already trying to help.

“They were freaking out,” he said, “pounding on the doors of the homes on either side.”

Most of the neighbors seemed to know that no one was at home in the house on fire.

Police Sgt. Dave Villotti had been at a nearby convenience store when he got the call. He was the first officer there and immediately sent word back — fire had overtaken the house.

“He evacuated the families on either side,” said police Lt. Chad Rensel.

At 109 South St., William Gold's family of four, including two children, made it out. At 113 South St., Meagan McMichael's extended family of eight, which also included a baby and other children, got out safely, too.

“It looked like they had been sleeping,” said Scott, who watched from his front porch as the next-door neighbors poured outside. “They were wearing night gowns and pajamas.”

Neighbors all were struck by how fast the fire spread.

Just minutes after the call, fire crews arrived. But by then, Codispot knew it was too late to save the house.

His priority was saving the houses on each side. Bystanders feared the worst.

“I thought both of the other houses were going to go up,” Bryner said.

“We pulled a line off the truck and tried to keep the flames from (109 South St.),” Codispot said.

Crews coated the neighboring home and the one at 113 South St. with compressed-air foam.

The efforts paid off as both homes sustained only minimal heat damage, fire officials said.

“The guys did one helluva job,” Codispot said.

There was no saving the house that caught fire.

Firefighters were limited to using a “surround and drown” strategy. Crews deployed hose lines to shower the front and back of the house with foam before eventually using water.

The Butler VA fire department assisted as did the Butler Township Volunteer Fire District 3, which brought its ladder truck.

“We attacked it from the front and the back,” Codispot said. “There was as much fire coming in back as there was in front.”

It took about an hour before the fire was contained enough to allow a crew inside.

But stubborn “hot spots” kept crews there until about 6:30 or 7 a.m.

Based on his observations and what neighbors told him, Codispot said it appeared the house exploded.

“All signs sure point to some type of explosion,” he said. “The entire back wall on the first floor and basement were gone, the side wall was buckled, parts of one of the front doors was in the front yard.”

Two other pieces of that same door were blown clear across the street.

“We did have the gas company here,” Codispot said. “They've checked everything. We're going to go from there.”

Police did an initial investigation but are awaiting the fire marshal's report.

I spoke to some people,” Rensel said, “and they confirmed there was a big boom and an immediate fire ball. I wondered what caused that — a gas leak or something gas related? I don't know.”

He minimized but did not entirely rule out another possible cause — a methamphetamine lab. Many neighbors openly wondered if there was a meth lab in the house.

“We heard talk about that,” Rensel said, “but once the fire was out we peeked around and didn't see anything suspicious or anything that would indicate a meth lab.”

Still, the house was too unstable to allow for a thorough inspection.