Up to 70 firefighters battled 20-foot flames and explosions that destroyed a shed at a Coram concrete and stone supplier just before midnight, fire officials said early Thursday.

Flames broke out late Wednesday in the massive yard of the Stevens Stone & Brick Co., inside a 40-foot-long cargo container that had been rehabbed into a shed used to mix cement, Coram Fire Chief Michael Fields said. People leaving the 7-Eleven next door reported the fire shortly after 11 p.m., he said.

When the first of firefighters from five departments arrived, they had to cut the front gate locks on the Route 112 entrance but were thwarted in reaching the blaze when their firetrucks were blocked, he said: “We took the gate off, only to find a humongous truck there.”

It was a 10-wheel tractor trailer about 50 feet down the driveway, the chief said, and that forced him and others to head to the back entrance, off Route 83, in search of the source of the fire.

“As I made my way to the back of the property, there were a couple of explosions,” Fields said. “We could have had a really bad situation there.”

He believes they were caused by acetylene tanks in the shed, which contained welding torches, a boiler and other equipment.

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Firetrucks with tower ladders helped get the blaze under control at 11:50 p.m. and Brookhaven Town fire marshals were still at the scene after 1 a.m. to sift through the wreckage for the cause of the fire, the chief said.

Investigators believe it may have been an electrical fire, Fields said, but they want to see if the shed and its use as a cement mixing area were legal, Fields said.

He said no one was injured in the fire.