The U.S. Marine Corps has released its official findings in the October 2025 artillery malfunction that rained shrapnel on Interstate 5 during a live-fire demonstration near Camp Pendleton — and investigators say the incident was the result of an extraordinarily rare ammunition defect.

A 666-page investigation report dated Dec. 19 and first reported Monday concluded there is "no definitive answer" as to why an M795 high explosive round detonated prematurely at about 1,480 feet over the freeway during a celebration of the Corps' 250th anniversary. The round was "manufactured to a tolerance of one defect in a million," the report states. No negligence or wrongdoing by Marine Corps members was found.

The malfunction occurred during the first volley of a planned 60-round live-fire demonstration attended by Vice President JD Vance and Defense Secretary Pete Hegseth. Two California Highway Patrol vehicles were struck by shrapnel. Nobody was injured. The remaining rounds in the demonstration were immediately canceled.

Investigators cited several possible contributing factors — including the howitzer guns being positioned too close together and the "potential presence of anomalous electromagnetic energy" near the weapons. The report acknowledged the combination of factors that day differed from "thousands" of previous successful uses of the same shell and weapons system.

I-5, which runs along the coast between Los Angeles and San Diego and passes directly adjacent to Camp Pendleton, was temporarily closed by California Gov. Gavin Newsom ahead of the demonstration over safety concerns — a decision that drew sharp criticism from the White House at the time. A group of 26 California federal lawmakers later wrote to Defense Secretary Hegseth demanding answers about who approved live artillery fire over a civilian highway corridor.