Defense

Marines: ‘One-in-a-million’ malfunction caused explosion over I-5

by Jackie Marquez - 03/16/26 2:34 PM ET

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SAN DIEGO, Calif. (KSWB/KUSI) — The U.S. Marine Corps (USMC) has determined that a “one-in-a-million” malfunction caused an artillery shell to explode over Interstate 5 and strike a California Highway Patrol vehicle with debris last year.

The incident occurred in October 2025 during an event celebrating the USMC’s 250th anniversary.

The findings come from a newly released Marine Corps report that’s over 600 pages long.

That report determined that the mishap was the result of a malfunction in the artillery’s fuze, which electronically controls the timing of detonation.

“The M767Al fuze is extremely reliable, with only one malfunction in its service history, in 2017, that was ultimately determined to be caused by an external factor,” the report said. “It is manufactured to a tolerance of one defect in a million.”

Despite prior reliability, the fuze detonated in midair, about 1,500 feet above the I-5, rather than in the designated training impact area.

So, what caused the rare malfunction?

The report ruled out wrongdoing by Marines, interference by other unmanned aerial systems and birds.

Instead, the report suggested that several typically harmless conditions may have aligned in a way that changed the likelihood of a malfunction.

According to the Marine Corps, these factors could have been the close proximity of weapons to each other, blast overpressure and the potential presence of an electromagnetic charge.

While no one was injured in the incident, the CHP has stated that a small piece of metal debris struck and damaged a CHP vehicle while it was working a freeway closure on the Las Pulgas Road on-ramp to I-5 South.

According to the Marine Corps report, that metal fragment was about 2 inches long and half an inch wide and left a small dent and scratch on the hood of the vehicle.

After the incident, CHP notified the Marine Corps, which then canceled additional live-fire exercises.

The closure of the I-5 was another contentious point during the military event.

In the report, the USMC states that initially, when President Donald Trump was set to attend the celebration, closure of the I-5 was a requirement. However, that requirement was lifted when the guest of honor was switched to Vice President JD Vance.

Even so, the CHP and Gov. Gavin Newsom implemented traffic breaks as a precaution.

“This was an unusual and concerning situation,” said CHP Border Division Chief Coronado. “It is highly uncommon for any live-fire or explosive training activity to occur over an active freeway. As a Marine myself, I have tremendous respect for our military partners, but my foremost responsibility is ensuring the safety of the people of California and the officers who protect them.”

Ultimately, while Marine Corps officials asserted that the malfunction was incredibly rare, the report recommended a review of the fuze and ammunition used in the demonstration, which may lead to changes for future demonstrations.

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