BROOKSVILLE — A fire that destroyed a lone cottage on Harbor Island on Friday morning, apparently started by a bolt of lightning, would have spread to the surrounding forest and destroyed it if there had not been torrential rains, a local fire chief said.

Matt Dow, chief of the Brooksville Volunteer Fire Department, said he was the first to arrive at the scene of the blaze. “It was completely involved,” he said. “It was just the platform of the house left.”

Within about five minutes, other firefighters arrived with equipment, ferried to the island on fishing boats and other vessels from the nearby Bucks Harbor Yacht Club.

“All we did was try to save the forest around it at that point,” added Dow.

The firefighters used several portable pumps to douse the scene with seawater.

“We were surrounding it and drowning … just trying to protect the rest of the forest at that point,” said Dow. The flames already had crept a short distance into the woods.

The firefighters worked during a torrential downpour and some wind. “I’m actually glad it was raining,” said Dow. “If it hadn’t been raining, the whole island would have gone up.”

The spruce tree forest was very close to the cottage, he noted. “It wouldn’t have taken a lot for the whole island to go.”

The cottage was the only house on the private island, which was acquired by the Maine Coast Heritage Trust in December.

“At this point, it looks like a lightning strike” was the cause of the fire, said Dow. A couple of nearby trees were “scored up” by an apparent lightning strike, he said. In addition, “One of the pillars the house was on completely exploded.” The house sat on concrete posts on a ledge.

A passer-by witnessed a possible lightning strike of the house, according to a Hancock County communications dispatcher.

It was low tide, which made it difficult both to load the boats with equipment and to disembark at the island and get the portable pumps ashore.

About 15 firefighters from Brooksville, Sedgwick, Deer Isle, Brooklin and the Maine Forest Service responded, according to Dow. They were aided by about five local fisherman who loaned the use of their boats.

“It worked out great,” said Dow. “These fishermen deserve a round of applause” for helping.

The fire department was summoned by the Hancock County Sheriff’s Office about 9:15 a.m., Dow reported.

Jane Arbuckle, director of stewardship for the conservation organization, described the building as a one-story cottage with a deck. “It’s a small summer cottage,” she said by phone from the organization’s offices in Topsham. Arbuckle did not know the value of the building.

Although it already purchased the island, the trust is in the midst of a fundraising campaign to retain possession of it, said Arbuckle. The organization was unsure how it was going to use the cottage.

“We’re lucky that nobody was hurt,” said Arbuckle.

Trust officials first received word of the fire about 9 a.m., she said.

A trust staff member in charge of the island was on the scene to assess and document the damage.

The house was empty, according to Arbuckle. “The former owners are removing some of their belongings,” she said. “They were supposed to be done this week.”

Harbor Island is a 25-acre, heart-shaped island in Penobscot Bay in the middle of Bucks Harbor.