No injuries were reported at Saturday's event at California's Camp Pendleton, which US Vice President JD Vance attended.

An artillery round "detonated overhead prematurely," damaging a highway patrol vehicle on a nearby freeway, the agency said in a statement.

"This was an unusual and concerning situation," said the patrol's division chief Tony Coronado.

He added that it is "highly uncommon for any live-fire or explosive training activity to occur over an active freeway."

California Governor Gavin Newsom had been critical of the live fire display in the days leading up to the event which closed off a 17-mile (27-kilometer) stretch of the Interstate 5 that links Los Angeles and San Diego.

Newsom, a frequent critic of the Trump administration, said on X that "this could have killed someone."

US media said the Marine Corps -- which had previously insisted there was no safety risk -- had also launched an investigation into the incident.

The event featured fighter jet flyovers, amphibious ship displays, explosions in a simulated village and Navy SEALS dropping into the Pacific Ocean from helicopters.

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