OGDENSBURG — A veteran state police trooper and a Canadian man were electrocuted Sunday in a freak accident while fixing a flat tire on an antique firetruck near the Ogdensburg-Prescott International Bridge.

Trooper Shawn W. Snow, 47, and an unidentified 51-year-old man were apparently killed instantly when the firetruck's ladder touched a live electrical wire, carrying a current through the truck, state police said. The international bridge was closed, with traffic diverted to the Thousand Islands and Seaway bridges.

"This type of accident is somewhat incomprehensible," Maj. Richard C. Smith Jr., commander of state police Troop B in Ray Brook, said at the Ogdensburg barracks. "It is a terrible day."

Trooper Snow was a 19-year veteran of the force and was assigned to the commercial vehicle enforcement unit. He was responsible for inspecting trucks that crossed the Ogdensburg-Prescott International Bridge or were traveling area roadways.

The Ogdensburg Police Department responded to a call at 4:45 p.m. with the city Fire Department for a report of a firetruck on fire with people underneath it near the bridge. Police said a firetruck heading to Canada was towing another antique firetruck when it had a flat tire on the road approaching the bridge. Trooper Snow volunteered to help replace the flat tire.

The trooper was under the truck at the time, fixing the tire. The truck's boom was extended to redistribute the weight from the flat right rear tire. When the truck was lowered, the boom hit a live wire, killing both men instantly, police said.

St. Lawrence County Coroner Russell B. Lawrence III pronounced the men dead at 5:27 p.m., believing the cause of death was electrocution. The deaths were accidental, he said. The bodies were taken, by a state police escort, to Claxton-Hepburn Medical Center. Autopsies were expected to be performed today by Dr. Michael Sikirica at St. Elizabeth Medical Center, Utica.

"It is almost undescribable," Mr. Lawrence said. "It is such a tragic thing."

The firetruck was from the Canadian Fallen Firefighters Foundation and was on its way back to Canada from a parade near Syracuse. The parade was sponsored by the Society for the Preservation and Appreciation of Antique Motor Fire Apparatus in America.

"It is such a tragic occurrence," said David M. Sheen, the Ontario representative for the Canadian Fallen Firefighters Foundation, in a phone interview from Toronto. "Our prayers and thoughts are with the families that were tragically involved in this unfortunate event."

A state police accident reconstruction team was investigating the incident. State police were assisted by the Ogdensburg Volunteer Rescue Squad, Ogdensburg Bridge and Port Authority and U.S. Customs and Border Protection.

The Canadian man had not yet been identified Sunday night because family members couldn't be reached. The larger firetruck had Quebec license plates and "Monti-Joli Region 4" on the door. The smaller truck said "Bytown, Ottawa."

"It is a tragedy for the law enforcement community tonight," Maj. Smith said. "The investigation is ongoing."