The Main Line mansion had been undergoing renovations for years, neighbors said. The homeowners were adding a new two-car garage, powder room, kitchen, family room, and walk-in closet to their six-bedroom, five-bathroom Gladwyne estate, according to renovation plans.

Then, around 8 Sunday night, came a sound and a shake that startled neighbors along Dodds Lane. In the darkness, the vacant house exploded, sending out a boom that was heard and felt across the region, prompting hundreds of 911 calls.

Police and fire officials gather around the leveled home on Dodds Lane in Gladwyne on Sunday night.
TOM GRALISH/STAFF PHOTOGRAPHER
Police and fire officials gather around the leveled home on Dodds Lane in Gladwyne on Sunday night.

A day later, questions remained about how and why the 9,000-square-foot home, which sold in 2016 for nearly $4 million, exploded. The incident was likely a gas explosion, authorities said, but it could take weeks to determine a cause. Officials would not speculate as to what preceded the blast, but stressed that residents should not fear for their safety. The incident, authorities said, was "very isolated."

"I don't see anything suspicious at this point," said Lower Merion Township Fire Chief Chas McGarvey. He said, however, that authorities had yet to enter the remains of the building. They spent Monday securing the area. The floor "was pancaked," he said, and rainy weather hampered their progress.

The home was owned by GF 2014, a limited partnership owned by Michael Grasso, a well-known developer in the area.

Debris can be seen in the trees after the home explosion on the 600 block of Dodds Lane in Gladwyne.
TOM GRALISH / Staff Photographer
Debris can be seen in the trees after the home explosion on the 600 block of Dodds Lane in Gladwyne.

Due to the size of the blast, the Bureau of Alcohol, Tobacco, Firearms, and Explosives was assisting in the investigation, as was the Pennsylvania State Police fire marshal.

"It's considered a crime scene until we find out what happened in there," McGarvey said.

The Montgomery County District Attorney's Office said Monday that it was not involved.

In nearby homes, residents reported broken windows and blown-in doors — residual damage from the blast, McGarvey said, but no one was hurt.

"It was lucky that there were no injuries," said Thomas Walsh, public information officer for Lower Merion. "This was a really large explosion."

Crews from Peco arrived in the neighborhood shortly after the explosion, immediately turning off the gas to that property before checking for any large-scale leaks that could affect the neighborhood. They found none, said Peco spokeswoman Afia Ohene-Frempong. On Monday, crews found a gas main leak nearby, which prompted officials to shut down a portion of Old Gulph Road between Mill Creek and Merion Square roads for emergency repair.

Yet "the level of this leak is not something anybody would anticipate causing an explosion," Ohene-Frempong said.

The scene at the house on Dodds Lane in Gladwyne where there was an explosion on Sunday night.
TOM GRALISH/STAFF PHOTOGRAPHER
The scene at the house on Dodds Lane in Gladwyne where there was an explosion on Sunday night.

"We can't even begin to speculate on the cause of the explosion," Ohene-Frempong said.

Joseph Grasso, 57, a developer and investor who owned the home until 2016, filed for bankruptcy in 2012. Four years later, he and his wife, Donna Grasso, 52, sold the property for nearly $4 million to his brother's limited partnership.

Joseph Grasso once owned Saxbys, a chain of coffee shops, and Center City's Union Trust steakhouse, which is now shuttered. Another relative, David Grasso, co-owns the restaurant Wm. Mulherin's Sons in Fishtown.

The scene at the house on Dodds Lane in Gladwyne on Sunday night.
TOM GRALISH / Staff Photographer
The scene at the house on Dodds Lane in Gladwyne on Sunday night.

Members of the Grasso family did not return requests made Sunday and Monday for comment.

Building materials hang in the trees at the scene a day after a gas explosion leveled the empty mansion.
DAVID MAIALETTI / Staff Photographer
Building materials hang in the trees at the scene a day after a gas explosion leveled the empty mansion.

When the explosion occurred, authorities said people were in a carriage home on the Grasso property and called 911. It was unclear who was in that adjacent property, which is across the driveway from the main house and appeared undamaged.

Those people did speak with authorities after the blast, McGarvey said. He said he did not know of any prior calls to the main home, which had been under renovations for an "extended" period of time.

A few doors down, Carolyn and Brendan Bovaird were settling into their living room chairs to watch the The Durrells in Corfu on PBS on Sunday evening when their house shook violently, rattling a tall, trapezoidal wall inlaid with window panes.

"These windows actually waved," Brendan Bovaird said Monday. Instinctively, he said, he jumped from his seat as his house shook, fearing the glass windows would break and fall on him and his wife.

Immediately after the couple felt the boom, Carolyn Bovaird called 911, but couldn't get through because police dispatchers were swamped with callers.

Other neighbors said they at first thought the noise may have been a meteor, a cannon blast, or gunshots.

The Bovairds walked out into the chilly night and up the road to the Grasso home, where they saw the extent of the damage, they said. Dozens of neighbors flooded out into the streets, which was soon packed with fire trucks, police cars, and news vans.

By Monday, as many residents began their work week, the streets remained congested with emergency personnel. At the end of a long driveway, investigators could be seen working on the property, much of which was full of debris. Fragments of the home appeared to be stuck in the trees.

Peco receives more than 20,000 calls per year about possible natural gas leaks, said Ohene-Frempong, but explosions from leaks are "rare, very rare."

Yet people who smell gas in a home should immediately get out of the property, find a safe place, and call Peco, she said.

Peco crews will remain in the area this week as a precaution.

"For an explosion of that size, to rock the land, you kind of just want to stick around," Ohene-Frempong said.

By the end of the week, authorities said, they expect to know more about what happened on Sunday night.

Building materials hang in a tree at the scene a day after a gas explosion leveled an empty mansion in Gladwyne.
DAVID MAIALETTI / Staff Photographer
Building materials hang in a tree at the scene a day after a gas explosion leveled an empty mansion in Gladwyne.