The Wethersfield firefighter was among crews from several towns trying to extinguish the blaze on Lamentation Mountain in Berlin, about 15 miles (24 kilometers) south of Hartford. Their name was not immediately released.
Berlin Fire Chief John Massirio said the firefighter was riding in a utility task vehicle on a trail when it rolled over near the Meriden town line. The cause was under investigation.
A procession of first responders was planned Tuesday evening to transport the firefighter from the mountain to the chief medical examiner's office.
The two other firefighters were treated at a hospital for minor injuries and discharged, Massirio said.
"The friendships run deep here and there's long lasting relationships between these people, so we're a tight-knit family," he said. "It's definitely felt throughout the surrounding towns. ... We're a family and it affects everybody here."
Fire departments across the state were posting their sympathies on social media.
The fire began Monday and crews have been working around the clock to contain and extinguish it. No evacuations have been ordered. The cause of the blaze is being investigated, officials said.
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