A rare malfunction sent explosives onto a highway near Camp Pendleton, the report said

The detonation of a military roundabout on the 5 Freeway during a training exercise at Camp Pendleton in October was a “one in a million” malfunction that investigators are struggling to explain, according to a US Marine Corps report released Friday.

The investigation found that the military round detonated before it happened because the device’s detector went off too early, and metal debris fell below. A California Highway Patrol cruiser was pierced with metal.

“Of all the days that this very unlikely event happened, why did this happen? What was different from the thousands of times before this event using the same shell-fuze combination, weapon system, and highly trained Marines?” read the report. “There is no definitive answer to these questions.”

The incident occurred on October 18 during the 250th anniversary of the Marine Corps, after Vice President JD Vance was escorted to Camp Pendleton to observe the mission. A 17km stretch of the highway was shut down half an hour before the work was due to start at 1:46pm, upsetting thousands of weekend commuters.

The White House initially criticized California Gov. Gavin Newsom for closing the highway during the military deployment, saying in X “The Marines have repeatedly said there are no public safety issues.”

But after word of the initial blast spread, Newsom called on the Trump administration to apologize, saying the operation was “reckless” and one of the White House’s “vanity jobs.”

“We thank the Marines for their thorough and thorough investigation — in stark contrast to the dangerous and practical demands of JD Vance and Pete Hegseth to shoot live ammunition in a public place for fun,” Diana Crofts-Pelayo, a spokeswoman for Newsom’s office, said in a statement.

The White House also did not respond to a request for comment.

According to the report, no one was injured in the fall, but it hit a CHP vehicle and a CHP motorcycle near Las Pulgas Road.

The report suggested that this may have been the cause of the premature firing but found that the soldiers were not at fault.

The explosives came from an M795 round fitted with an M767A1 fuze, which was intended to detonate the round electronically at a predetermined point, the report said.

Instead, the round prematurely exploded 453 meters, or about 1,486 feet, in the air, about 3,300 meters from the detonation point, according to the report.

The size of the shrapnel that hit the CHP vehicles was described as 2 inches by 2½ inches.

The report says the fuze used in the operation is “highly reliable” and had only one recorded malfunction, in 2017.

The report states: “It was designed to be able to tolerate one defect in a million. “The probability of the M767A1 malfunctioning is a statistical anomaly that was beyond any reasonable probability.”

Investigators ruled out the possibility that a round had hit a drone in the area and, although it may have hit a bird, noted that “no one was observed in the flight path of the projectiles at the time of the incident, and no biological remains were found in the shell fragments.”

Instead, the report noted that the M777 Howitzers, the weapons used for firing, may have been too close together when they fired, or the fuze was exposed to “extraordinary electrical energy in the area.”

According to the report, the positions of the Howitzers were changed three days before the event, using a 50 x 50 meter box which was smaller than what was planned. That meant some of the guns were 10 to 15 meters away, according to the report.

“Many police officers noted that they had never seen how close the howitzers were to each other but they also admitted that there is no fixed distance for gun placement in relation to artillery fire,” the report reads.

The report says investigators have reached out to the United States Secret Service to ask if electronic countermeasures, or ECM – which can be used to disrupt or jam electronic systems – were used by the agency that day during the demonstration. The White House military office told investigators it did not take these steps.

According to the report, President Trump was originally supposed to attend the event but Vance was sent instead.