Tuesday's three-alarm fire started with either a faulty extension cord or from a water cooler plugged into the cord, Watertown Fire officials said.
The fire started with the faulty cord in Lony Nail Salon, went into the basement before ending up in the ceiling where it was trapped for some time before bursting into heavy flames, said Capt. Stephen Igoe, fire inspector for the Watertown Fire Department.
"It seems like it was in the nail shop," Igoe said. "An extension cord had been cut or shorted out. Or, the bubbler that was plugged into it did. It was accidental."
The fire was first reported by a tow truck driver who spotted some smoke coming from the building at 309-315 Main Street, just west of Watertown Square. When firefighters arrived, smoke was beginning to pour out, Igoe said.
The building had a number of ceilings, including an old tin ceiling, which trapped the heat and gases, Igoe said.
"Buildings like that, which have been around for a long time, often have changes that include adding more ceilings," Igoe said. "That locks everything in."
Firefighters had broken windows so they could spray water onto the fire.
Then, an explosion knocked over some of the firefighters, and injured one. Igoe said that was likely due to the gases pent up in the ceiling.
"The best we can figure is we opened the front, and 15 minutes later it flashed (exploded)," Igoe said. "All the heat and gases were contained in the ceiling and all the super heated gases were oxygen starved and when it had access to oxygen it ignited the gases."
The fire damage was largely contained to the ceiling, with a small area where the extension cord was located also suffering burns, Igoe said. The rest of the building suffered water damage.
The exact cost of the damage has not been released yet, Igoe said.