A 155-millimeter shell fired during a live-fire demonstration for the 250th anniversary of the Marine Corps at Camp Pendleton on Saturday prematurely detonated, dropping fragments of the shell on a California Highway Patrol vehicle and motorcycle that were part of Vice President JD Vance’s protective detail, according to a patrol report.

No officers were hurt in the mishap, which dropped shrapnel onto the vehicles parked on a ramp to a major freeway that had been ordered closed by Gov. Gavin Newsom. The governor had objected to the plan to fire over the freeway, Interstate 5, and ordered a 17-mile stretch closed — against the guidance of military officials, who had said it was safe for it to remain open.

According to the patrol report, one officer described what sounded like pebbles hitting his motorcycle and the area around him, and two others saw a two-inch piece of shrapnel hit the hood of their patrol vehicle, leaving a small dent. The report says shrapnel was also found on the road near the motorcycle.

Mr. Newsom had warned that the Marine Corps’ plans to fire artillery shells over Interstate 5, the West Coast’s main north-south artery, could pose hazards for motorists on the stretch between Los Angeles and San Diego. The closure he ordered on Saturday caused significant backups on the portion of the interstate, which is used by approximately 80,000 people daily.

“We love our Marines and owe a debt of gratitude to Camp Pendleton, but next time, the vice president and the White House shouldn’t be so reckless with people’s lives for their vanity projects,” Mr. Newsom said in a statement to The New York Times.

Lt. Col. Lindsay Pirek, a spokeswoman for the First Marine Expeditionary Force at Camp Pendleton, said the Corps was aware of the report of a possible airborne detonation, and an investigation was underway.

“We are committed to determining the incident’s root cause and applying findings to future missions,” Colonel Pirek said. The statement did not provide additional detail on the munitions used during the exercise.

A spokesman for Mr. Vance declined to comment and referred reporters to the First Marine Expeditionary Force.

According to the report filed by the California Highway Patrol, the artillery round was fired at 1:46 p.m. from White’s Beach, approximately three-quarters of a mile south of Las Pulgas Road, where the highway patrol officers were parked after escorting Mr. Vance to the event.

The exercise — which the report said was expected to include the firing of approximately 60 155-millimeter shells — was terminated after the round prematurely exploded, the patrol said. Patrol officers checked the interstate for shell fragments but found none, and declared it clear before reopening it to vehicular traffic at 2:20 p.m.

The demonstration was part of a larger exercise marking the Corps’ 250th anniversary, attended by Mr. Vance and Defense Secretary Pete Hegseth. A Marine spokesman accompanying reporters witnessing the exercise was pulled aside by another Marine after the initial volley was fired by M777 howitzers, then told members of the news media that the initial plan for multiple volleys had been scaled back. The spokesman, Lt. Col. Brian Coleman, noted that there had been a lengthy back and forth between officials in the days leading up to the event, but gave no further information.

The decision to fire live artillery shells from the oceanfront training area was described as unusual by an active-duty Marine artillery officer and a former Marine artillery noncommissioned officer who spoke on the condition of anonymity because they feared reprisal. Those Marines said the only howitzer training they had previously observed at Camp Pendleton had taken place at approved artillery ranges on the main side of base, east of the interstate, which they said were a much safer option for training.

A highway patrol official based in the area also described it as an “unusual and concerning situation.” Tony Coronado, the highway patrol’s border division chief, said in a statement, that “it

is highly uncommon for any live-fire or explosive training activity to occur near an active freeway.”

Such mishaps are exceedingly rare, according to an active duty Marine who has spent more than 20 years as an artillery officer. The Marine, who was not authorized to speak publicly, said that the most likely explanation for such a mishap was a fault in the projectile’s fuze — a mechanical or electromechanical device fitted to the nose of the shell that causes it to explode.

The Marines fired howitzer shells across Interstate 5 on Friday evening as a rehearsal, evidently without incident.

The Marine Corps said before the event that the exercise would occur on “approved training ranges and comport with established safety protocols” without the need to close public roads.

“All air, surface and ground movements are scripted and rehearsed in accordance with standard operating procedures and established safety checklists,” the Marines said.