TROY — Flames ripped through a metal warehouse on Forest Avenue Wednesday afternoon in the city's second large commercial fire of the day.

Firefighters were called to the warehouse, off Pawling Avenue in the city's southeastern corner, around 5:10 p.m. When they arrived the structure was fully engulfed.

"There were major explosions throughout the building," Troy Fire Chief Eric McMahon said.

Because of the explosions firefighters were forced to back away and let the building fall, the chief said. Once it was down, crews moved back and extinguished the flames from the outside.

The bulk of the fire was out by 6:30 p.m.

Although no firefighters were injured, as of 7 p.m. firefighters had not yet gone inside to search for victims and don't know if anyone was inside when the flames erupted, McMahon said.

The property owner, Bill Hauet, said the building was used for storage, mostly housing old machinery, and no one works there. He said he hasn't been inside the building for a few years.

"Any tank can create an explosion," McMahon said. "It could have been any number of things."

McMahon said he doesn't know yet what caused the blaze and will ask the state to assist in the investigation.

Originally the fire department contemplated bringing in more crews but, given those issues and the fact that the structure was starting to collapse, they fought the fire exclusively from the outside, McMahon said.

"We have limited access for vehicles. We have limited access for crews. We have water problems,"

Earlier that day, the Troy fire department fought another massive fire that destroyed the Alpha Lanes bowling alley on Fifth Avenue. A cause for that fire has not yet been determined.

Read more: Fire consumes Troy bowling alley

Crews were still at the Fifth Avenue scene when some were called away to the Forest Avenue fire.

Several city firefighters battled both blazes Wednesday and said at 7 p.m. that they were exhausted. They had showered and eaten between the two fires but still had 13 hours left on their shift.

"They've had quite the day," McMahon said. "We're hoping the rest of the day goes a bit easier."

He said there is some concern about having enough time to properly decontaminate the firefighters' turnout gear, which must be done after every fire.

"It is going to be challenging for sure, to get all of the gear cleaned, to get it back in service, to get it to the firefighters out in the stations in time for their next shift," he said. "This will show where the weaknesses are in our plan."