An explosion at the Long Beach Fire Department’s training facility last month was caused by a buildup of natural gas in a basement where recruits were getting used to how their protective gear works when faced with high heat and flames, according to an internal memo authored about a week after the accident.

The memo, which the Long Beach Post obtained Wednesday through a public records request, happened during an initial live-fire training exercise for a group of recruits at the Fire Department’s training facility at 2249 Argonne Ave. on Nov. 8.

During the exercise, a natural gas line feeds controlled flames in the basement of a training tower so fire recruits can get a sense of how their gear protects them while performing tasks with prop equipment.

According to the memo, four fire recruits and five instructors were in the basement when the flames went out for some reason.

“Personnel were immediately ordered to exit the structure,” the memo says. “While exiting, the natural gas was re-ignited, causing an explosion to occur in the basement.”

The blast was powerful enough that it blew out doors and windows and sent debris flying out of the tower.

All the recruits and trainers were taken to hospitals or urgent care centers to make sure they were OK, but all of them were back to work within the day, according to LBFD spokesman Capt. Jack Crabtree.

Since the explosion, the tower has not been used in any live-fire training. The LBFD is waiting for contractors to assess and repair the gas and electrical systems.

The department is still investigating why the flames went out and then re-ignited.

A more in-depth report with recommendations about how to avoid similar situations in the future is expected to be published in January, the memo says.

Explosion during LBFD training exercise leaves 11 with minor injuries, department says