GRANTHAM — Firefighters battled a blaze Monday night that injured two people, one critically, and gutted a home.

Grantham Fire Department’s engine arrived at 505 Route 10 South, a mostly-wooded area across from a public park and about two miles from Grantham’s commercial center, at about 7:50 p.m. to find the 950-square-foot house fully engulfed, with flames showing through the roof.

Zachary Tyler Beaulieu and his girlfriend, Becca Wheeler, were staying in a small apartment on the second floor of the home at the time of the fire. Beaulieu’s grandparents, Donna and Normand, own the house and live next door.

Donna Beaulieu said in a telephone interview Tuesday that her grandson mistakenly thought that Wheeler and the couple’s dog, who had escaped the building, were inside still and went back in the burning house to find them. He sustained severe injuries before jumping out a second-story window.

“He was burned bad,” she said. “When he walked in the house, I thought he was a zombie.”

The Dartmouth Hitchcock Advanced Response Team, or DHART, helicopter was deployed immediately due to the nature of Beaulieu’s injuries, according to Grantham Fire Chief Justin Hastings. A Lebanon ambulance brought him to the landing zone and then DHART took him to a hospital in Boston, where he remains in “critical but stable condition,” according to his grandmother.

New London Ambulance later transported Wheeler to Dartmouth Hitchcock Medical Center in Lebanon, where she was treated and released for minor injuries. The dog was unharmed.

The cause of the fire is unknown at this time, Hastings said. The home was “so badly damaged that it was hard to determine” how it began, he said.

He contacted the New Hampshire fire marshal, which is a requirement any time a fire results in an injury, Hastings said.

The property was appraised in 2022 at $180,400.

In addition to its ambulance, Lebanon responded with an engine and crew, and Lebanon Chief James Wheatley reported in a telephone interview Tuesday afternoon that “there were some water supply challenges due to the rural location,” but that there were no other complications to the suppression effort. It took about an hour to get the fire under control.

Grantham’s firefighters are “paid-on-call,” meaning that they are compensated only when they respond to a fire or attend training. Grantham’s fire department has 15 members, seven of whom responded Monday night.

“I was very happy with our response,” said Hastings. “Manpower was not an issue.” He noted also that Grantham is “lucky with our mutual aid,” and that other departments responded quickly.

In addition to the Grantham and Lebanon fire departments, crews from the neighboring towns of Croydon, Springfield, N.H., New London, and Sunapee also responded to the fire.

Christina Dolan can be reached at cdolan@vnews.com or 603-727-3208.