AMHERST – The precise cause of Friday’s 3-alarm fire in Amherst, which neighbors said was punctuated by a loud explosion that shook their homes, continues to be the focus of the ongoing investigation by local and state authorities.

Firefighters from several communities assisted Amherst crews in containing the heavy flames that erupted around 9:30 a.m., and pretty much engulfed the large home by the time the first due crews arrived.

As for the address of the home, Amherst fire Chief Matthew Conley reported it as 13 Clark Ave., one of a half-dozen or so residences on Clark Avenue, which runs off of Broadway and comes to a dead end just south of Washer Cove.

Just on the other side of Washer Cove is another road called Clark Island Road, which leads from a road called Embankment Road onto Clark Island.

Meanwhile, Matt Benoit, who was interviewed by news outlets at the scene, said he was doing some work on the roof of a nearby residence when the flames broke out.

“It shook the whole house,” Benoit said, referring to the effect the explosion had on the house he was working on.

“It got so hot, so fast,” he added.

Conley, the fire chief, said the first-arriving crews immediately went to work to make sure nobody was still inside.

A second alarm was sounded once the size and severity of the fire was realized, and a third alarm followed in short order.

According to Anthony Booth, an investigator with the state Fire Marshal’s office, and district chief Adam Fanjoy, contractors were working at the house when the explosion occurred and flames broke out.

Those inside at the time were able to escape the burning building, but one of the contractors sustained burn injuries to his forearms, Booth said.

He said the unidentified man was treated at the scene by Amherst EMS personnel, transported to a local hospital, then transferred to a Boston area hospital for additional treatment.

Booth said the man is listed in stable condition.

The house had been under construction for some time, according to neighbors and social media posts, and the work was nearing completion.

The force of the explosion launched newly-installed windows, doors and other construction materials into Baboosic Lake, where they could be seen floating around the cove as firefighters overhauled the smoldering rubble checking for hot spots.

When it became apparent the house couldn’t be saved, firefighters switched to defensive operations, in which they concentrated on preventing the fire from spreading while protecting nearby buildings and other possessions known as “exposures.”

Assisting Amherst crews either at the scene or by covering the town’s fire stations were crews from Nashua, Hollis, Merrimack, Milford, Mont Vernon, Bedford, New Boston and Manchester.

Dean Shalhoup may be reached at 594-1256 or dshalhoup@nashuatelegraph.com.