POINT MACKENZIE -- A tanker trailer leaking liquefied natural gas likely caused last week's explosive fire at a Point MacKenzie plant, state investigators say.

They still don't know what caused the leaking gas to ignite in the first place, however.

No one was injured after three explosions and a fire destroyed a 3,000-square-foot maintenance building at the Fairbanks Natural Gas Co. liquefied natural gas plant last Thursday. The ensuing fire burned so hot that firefighters were forced briefly to move a quarter mile away. The blaze quickly burned itself out.

The plant shut down that day but resumed production late Thursday night. The production facilities were located away from the maintenance building.

The plant super-chills Cook Inlet natural gas into a liquid form that then is trucked to Fairbanks.

The state fire marshal's office is still investigating last week's incident. Investigators remained at the plant Monday, their third day at the scene.

The tanker itself never exploded. But, contrary to initial reports, it had not been purged of its contents before being brought inside the maintenance building for repairs, state fire officials said.

"The tanker was not empty," said Kelly Nicolello, assistant state fire marshal. "It was brought down to zero pressure, which leaves plenty of material still in the tank."

The tanker -- built like "a big thermos bottle basically" -- was in the shop due to a vacuum leak in its shell, Nicolello said.

The gas is normally stored below minus-260 degrees to keep it in a liquid form. Unpressurized, and in the warm confines of the shop, the contents of the tanker converted back to gaseous form, Nicolello said. It wasn't clear where the gas leaked from.

Fairbanks Natural Gas president Dan Britton was not available for comment Monday.

Britton said last week the company believed a number of waste oil barrels in the building caused the explosions.

The fire marshal's office still has no idea "what caused the air-gas mixture to explode like that," said Nicolello.

The agency normally investigates residential or commercial fires, not industrial accidents. So the state is working with Fairbanks Natural Gas and its insurance company, Nicolello said. "We're learning as we go along. We don't claim to be experts. That's why we're working with the insurance company, because they have people available who are."

Possible ignition sources include lighting or heating systems and any equipment running at the time of the blasts, he said.

A maintenance employee told Mat-Su fire officers last week that he was reconfiguring equipment after a power failure when he heard a buzzing sound, then a boom. He rousted a napping truck driver, then called 911.

The plant serves the utility's roughly 1,100 customers in Fairbanks North Star Borough. Trucks make 30 to 40 deliveries each week.

For some Mat-Su residents, however, last week's incident might have been the first they heard of a plant that's operated since 1998 without much notice in a sparsely populated area near the Point MacKenzie Correctional Farm.

"Folks don't even realize what that facility does," said Jeff Wendt, a former plant operator at the facility who with his wife owns the Point MacKenzie General Store some miles away. "It's at the end of the road. It's been out there for a good number of years. But if it's not in your backyard, nobody knows anything about it."