SALEM — A 26-year-old man was seriously injured yesterday when he fell from the roof of a construction site on Highland Avenue.

Justin Cormier of Gardner was "unconscious and unresponsive" after falling about 20 feet to the cement floor of the Tri-City Sales building under construction at the corner of Marlborough Road, according to police. Last night, Cormier was in critical condition at Massachusetts General Hospital, a spokeswoman said.

Cormier suffered what appeared to be severe head injuries in the 12:19 p.m. accident, police said.

"It just appears to be a very unfortunate and sad accident," Detective James Page said.

Cormier may have been installing a flashing panel for a roof air conditioner, according to a construction official.

"It is my understanding he had removed a safety cover over an air-conditioning cutout and fell through it," said Tom Groom of Groom Construction of Salem, the general contractor.

Cormier was employed by Multi-State Roofing of Winchendon, a subcontractor, according to Groom. The Salem News was not able to reach a company spokesman.

No other workers on the roof saw him fall, according to Page.

"We don't know what happened up there," he said. "We just know from reports from workers inside that he did fall through the roof."

No foul play is suspected, Page said.

The victim was treated at the scene by Patrolman Dennis King, who was working a road detail next to the construction site when someone came running outside asking for his assistance.

As soon as the officer saw Cormier, he called for a medical helicopter.

The man was taken by Northshore Ambulance to Salem Hospital, then flown to Mass General in Boston.

Groom is constructing a new building for Tri-City Sales, an appliance and electronics store that has been in business for almost 50 years.

The building, which will include a Dunkin' Donuts, is scheduled to be completed in the fall. A second building for a CVS pharmacy will also go up on the site.

Last August, a 21-year-old Lynn man died after falling from the roof of the First Baptist Church on Lafayette Street. A roofing company was replacing old slate tiles with shingles as part of a building renovation.