BUFFALO, N.Y. — A 37-year-old Buffalo firefighter died battling a four-alarm structure fire Wednesday, Buffalo Mayor Byron Brown said.

They were unaccounted for at one point, believed trapped by falling debris from a blaze that led to an explosion that sent flames and smoke billowing into the street.

Video recorded by bystanders showed teams of firefighters outside the building being knocked over and enveloped in a cloud of ash, then flailing to regain control of spraying hoses.

Rescuers were trying to reach the spot where the missing firefighter was believed to be trapped, officials said.

“This is a difficult day in the city of Buffalo, and certainly we are asking for prayers,” Brown said

Firefighters responded to the building, which held a theatrical costumes shop, in a downtown commercial district at about 10 a.m., Fire Commissioner William Renaldo said.

“Quickly thereafter there was a mayday call,” Renaldo said. “There was a structural collapse, an interior collapse. Firefighters had to evacuate.”

Renaldo said at an earlier briefing that firefighters had not had any contact with their missing colleague since the firefighter became trapped.

The cause of the fire was under investigation. Renaldo said his department was investigating a report that workers had been seen with torches near the building, which was being renovated.

The Associated Press contributed to this story.