A Marine investigation found a faulty electronic fuze led to the premature detonation of a high explosive artillery shell over Interstate 5 during an October Marine Corps birthday event with Vice President JD Vance.
The fuze malfunction was "highly improbable," the investigator wrote, but ruled out negligence or wrongdoing by the Marines tasked with handling and firing the munitions.
The investigation was released by the Marine Corps Friday.
The decision of whether to close the freeway was a matter of contention between state and federal officials in the days leading up to the Oct. 18th event at Camp Pendleton.
On Friday, Oct. 17, Marines fired shells across traffic on I-5 during rehearsals for the event, shown in videos published — and subsequently unpublished — by the Pentagon.
On Saturday, Gov. Gavin Newsom ordered the freeway closed for the duration of the artillery shoot.
Shrapnel from the exploded shell hit a California Highway Patrol vehicle parked on the interstate as part of Vance's motorcade.
According to the investigation, a fault in this model of fuse is a "one in a million" risk.
Doug Krugman is a retired Marine lieutenant colonel who led infantry units in Iraq and Afghanistan. He said he's planned training and combat missions with Howitzer fire and has never seen this type of malfunction.
"That type of fuze functions correctly better than 99.9% of the time," Krugman told KPBS Monday. "But in this case, one in 1 million is still greater than zero."
Investigators found only one other incident of a shell exploding prematurely — that one after striking a bird.
But the investigation ruled out bird strikes or a collision with anything else in the air — including other Howitzer shells.
Investigators said the arrangement of the guns on Red Beach was unusual, but not outside regulations.
Six Howitzers were arranged in a space about 35 meters long, the investigation says.
Investigators said the close proximity and simultaneous fire could have led to the kind of electromagnetic interference that might have caused the fuze to malfunction, but they had no way to test their hypothesis.
Krugman was promoted to colonel before retiring from the Marines in protest of President Donald Trump last year. He wrote about his experience in the Washington Post.
Despite moral and political differences, Krugman said as an infantry officer he takes issue with the tactics of the October demonstration.
"In modern warfare, putting six artillery pieces in a ... 35 meter long stretch on a beach ... would be tactically extremely unsound and a horrible idea," he said.
Krugman said the entire demonstration served as a photo op for the Trump administration.
"The weapons were packed that close together and they were shooting from that beach to make sure it was a good photo opportunity," he said. "Whether or not that's a wise use of government resources (is) a question the voters can answer in the future."
Base range control officials told investigators artillery hadn't been fired from that beach since the late 1950s — and, until Oct. 17, never over traffic on I-5.
