BUFFALO, N.Y. >> A firefighter died inside a burning commercial building in downtown Buffalo, New York, on Wednesday, where a fire and sudden blast 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. Fire Commissioner William Renaldo said the fireball was likely caused by a backdraft, which he said occurs when oxygen is “sucked into the building and then blown back.”

The firefighter was reported missing shortly after crews arrived at the building, which held a theatrical costumes shop. Renaldo said the situation quickly deteriorated and firefighters had to evacuate to escape the collapsing interior.

“The rest of our firefighters on the scene were able to safely evacuate and unfortunately, he was not able to,” he said at an afternoon news conference.

The firefighter’s identity was not immediately released.

Rescuers initially held out hope that the missing firefighter would be found alive as crews tried for hours to reach the spot where he was believed trapped. Eventually the scene turned grim and firefighters stopped to salute as an ambulance drove away from the scene.

Mayor Byron Brown confirmed the death Wednesday afternoon.

“I am deeply saddened to report that a member of the Buffalo Fire Department has lost his life while bravely fighting the four-alarm fire on Main Street in Downtown Buffalo today,” he said in a statement. “I ask everyone to keep the family and fellow firefighters in your prayers.”

Brown ordered flags on city buildings lowered to half-staff.

The cause of the fire is under investigation. Renaldo said workers were using blowtorches on the building’s exterior before the fire and that heat may have transferred through brick or mortar to the interior and ignited paper or other combustibles.

“It looks like a possibility that the fire was smoldering for some time inside the building before anyone was made aware,” he said.

The building is owned by former U.S. Rep. Chris Jacobs and was being renovated. Brown said inspectors determined after the fire that it was unstable and would be immediately demolished.