WASHINGTON — For want of a secure bracket, the F-35B weapons bay caught fire.

That line may not have been included in the famous "for want of a nail" proverb, but it explains an October mishap in which a fire broke out on a Marine Corps F-35B during a training flight at Marine Corps Air Station Beaufort in South Carolina.

According to Air Force Lt. Gen. Christopher Bogdan, who leads the F-35 joint program office, the root cause of the fire was a loose bracket, which —when functioning properly — holds electrical wires away from other components in the weapons bay. The bracket became loose, causing the wiring to move and chafe at its own insulation, creating a spark. Those electrical wires sit next to hydraulic lines and other flammable parts, further increasing the risk of a fire.

Thankfully, the pilot landed the aircraft safely and was not injured. F-35B flight operations have continued despite the mishap.

"We knew about this problem long before that [incident], and all of our airplanes were being retrofitted with a new bracket," Bogdan said during a Dec. 19 briefing to reporters.

The engineering work for the bracket retrofit has been finished, but the aircraft affected by the fire, assigned to Marine Fighter Attack Squadron 501, had not been modified with the new hardware yet. Instead, aircraft with the original brackets underwent periodic inspections to check whether the bracket is still sturdy.

"It had passed its previous inspection, but the bracket still became dislodged," Bogdan said, adding that a new inspection regiment has now been put into place as a result of the mishap.

The October incident was the F-35B’s first Class A mishap, which involve loss of life or damage of more than $2 million, reported Defense News sister publication Marine Corps Times.

Because not all aircraft have been modified, B-model pilots are flying with heightened levels of risk, Bogdan acknowledged.

"Yes, there are acknowledged risks, and that would be one of them," he said. "And until you fix that bracket, every airplane in the B-model that doesn’t have that bracket is going to have to be inspected, and hopefully that bracket remains in place when it’s flying."

Just a month earlier, an F-35A caught fire while still on the ground at Mountain Home Air Force Base, Idaho. According to an Air Force statement, the fire began emanating from the aft section of the aircraft and was quickly extinguished.

The root cause of the September incident has not yet been made public, but no aircraft were grounded as a result.

More details will likely come to light in the coming months. Lt. Gen. Darryl Roberson, head of Air Education and Training Command, decided this month to start an accident investigation board (AIB), a service spokesman confirmed to Defense News on Tuesday. Usually an AIB is initiated after initial safety investigations conclude, and the process results in a report delineating the cause of a mishap.