WASILLA -- The State Fire Marshal's office has determined the cause of a fiery explosion last month at a Point MacKenzie liquefied natural gas plant.

No one was injured after three explosions and a fire leveled a 3,000-square-foot maintenance building at the Fairbanks Natural Gas Co. plant on Dec. 17.

The cause: gas leaked from a tanker trailer under repair, pooled at the maintenance-shop ceiling, then ignited after contacting an arcing fluorescent light, according to a finding released late last week by Deputy Fire Marshal John Bond.

Investigators also determined that the shop was constructed under building and fire codes that don't allow for work on hazardous materials such as natural gas.

That kind of work falls under an occupancy rating that generally requires encased lighting fixtures and gas monitoring equipment that could have prevented the fire, Bond said.

Fairbanks Natural Gas faces no enforcement action from the fire marshal's office.

The company was unaware of the building's occupancy rating, said president Dan Britton. It will follow all building requirements when it replaces the shop, Britton said. "Incidents happen," he said. "We don't flaunt the law. When things are required we change them."

The company super-chills Cook Inlet gas into a compressed, liquid form at the facility, then transports the liquefied gas by tanker truck to roughly 1,100 customers in Fairbanks North Star Borough. The plant is located in a sparsely populated area near the Point MacKenzie Correctional Farm.

The day of the explosion, a maintenance worker said he was reconfiguring equipment after a power failure when he heard the first blast. Mat-Su firefighters arrived to find the tanker surrounded by flames and gas igniting as it spurted out. They tried to extinguish the flames, but soon backed off for safety and let the fire burn itself out.

The blast destroyed the shop, blowing off walls, though the tanker never exploded. Several 55-gallon drums of waste oil stored in the building also ignited, firefighters said.

The shop was built by a previous owner in 1995, Bond said. Fairbanks Natural Gas began operations at the plant in 1998. It appears state fire marshals -- responsible for enforcing building codes -- had not conducted an official visit to the plant since then, apart from one informal tour.

Marshals usually examine a building if a business like Fairbanks Natural Gas plans new construction, a remodel or a new use for an existing building. Marshals do what's called a plan review.

The state fire marshal's plan review bureau didn't visit the plant until they participated in a May 2008 tour, when they happened to be out at Point MacKenzie on another job, said bureau supervisor Carol Olson. The team saw tankers outside, but "we didn't realize they were actually putting them inside the building," Olson said.

Officials from Alaska Occupational Safety and Health are also investigating the incident, said the agency's enforcement chief, Steve Standley.

The investigation -- his agency's first at the LNG plant -- will take another week, Standley said.