No injuries were reported, but the vehicle was damaged, CHP said Sunday in a statement.
The live-fire event at Camp Pendleton was part of the U.S. Marine Corps' 250th Amphibious Capabilities Demonstration at Red Beach.
The incident occurred over a stretch of the I-5 where CHP officers were supporting a traffic break during the live-fire training demonstration.
The live shell was said to have detonated prematurely mid-air.
The Marine Corps has been notified of the incident, and additional live-fire demonstrations were canceled, CHP said.
"This was an unusual and concerning situation," CHP Border Division Chief Tony Coronado, who identified himself as an active Marine, said in a statement. "It is highly uncommon for any live-fire or explosive training activity to occur over an active freeway."
The demonstration involved elements of I Marine Expeditionary Force and U.S. Third Fleet, highlighting the Navy-Marine Corps' "ability to project combat power globally, from ship to shore, with speed and precision," the I Marine Expeditionary Force said Saturday in a statement.
"The CHP has filed an internal report on the incident, with a recommendation to conduct an additional after-action review into the planning, communication and coordination between federal, state and local governments around the event on Saturday, October 18, to strengthen protocols for future demonstrations and training events near public roadways," it said.
Spokesperson Capt. Gregory Dreibelbis told CNN in a statement that the Marine Corps was investigating.
"We are aware of the report of a possible airborne detonation of a 155mm artillery round outside the designated impact area during the U.S. Marine Corps Amphibious Capabilities Demonstration," Dreibelbis said.
"The demonstration went through a rigorous safety evaluation, and deliberate layers of redundancy to ensure the safety of fellow citizens," he added. "Following established safety protocols, firing was suspended."
California Gov. Gavin Newsom had closed the section of I-5 ahead of the event as a precaution.
Ahead of the event, the Democratic governor lambasted President Donald Trump for scheduling the demonstration over the civilian transportation route.
"This president is putting his ego over responsibility with this disregard for public safety," he said in a statement. "Firing live rounds over a busy highway isn't just wrong -- it's dangerous. Using our military to intimidate people you disagree with isn't strength -- it's reckless."
Newsom, a potential Democratic presidential candidate, and Trump have been in a very public feud for years. During Trump's second term, Newsom has especially targeted Trump with criticism over his immigration policies and deployments of the military to Democratic-led states.
On Sunday, Newsom said, "this could have killed someone."
"This is what the White House thought was fine to fly over civilians on a major freeway," the California governor's press office said in its own statement on X, which included a picture of a soldier carrying the large munition on his shoulder.
"Thankfully, the Governor closed it."
Major California freeway shut amid US military live-fire exercise
Los Angeles, United States (AFP) Oct 19, 2025 -
A major US freeway in California was shut Saturday due to the US Marine Corps firing live artillery over the roadway as part of ceremonies marking its 250th anniversary, an event attended by Vice President JD Vance.
The closure of a 17-mile (27-kilometer) stretch of Interstate 5, which links Los Angeles and San Diego, snarled traffic for hours and set off another spat between California's liberal governor and Republican Donald Trump's White House.
"The President is putting his ego over responsibility with this disregard for public safety," said Governor Gavin Newsom, a frequent Trump critic who is expected to make a White House run in 2028.
"Firing live rounds over a busy highway isn't just wrong -- it's dangerous."
Signs near the interstate warned on Saturday: "Live weapons over freeway."
The order to shut the freeway came after California Highway Patrol officials warned that live munitions flying overhead would distract drivers on the oceanfront stretch of Interstate 5 near Camp Pendleton.
In a statement, the Marine Corps insisted there was no risk to the public.
"Artillery pieces have historically been fired during routine training from land-based artillery firing points west of the I-5 into impact areas east of the interstate within existing safety protocols and without the need to close the route," it said in a statement.
"This is an established and safe practice."
The massive Marine exercise featured fighter jet flyovers, amphibious ships, explosions in a simulated village and Navy SEALS dropping into the Pacific Ocean from helicopters.
In his address, Vance said the Trump administration was focused on supporting Marines and removing "woke" priorities that he argued have weakened the US armed forces.
"When officials try to shift focus to mandating diversity quotas, or they try to inject partisan politics into the American armed forces, they impede the Marine Corps's ability to do its best work.
"And that's why the secretary of war and the president of the United States have stood so firmly against that crap," Vance, a Marine veteran, told the assembled troops.
The Marine display came the same day that millions took to the streets from coast to coast in the United States to protest the hardline policies of the Trump administration, which have included the dismantling of diversity and equity programs.
In June, Trump ordered National Guard troops to Los Angeles to support federal officials in carrying out sweeping immigration raids and to tamp down local protests.
The deployment, which also included hundreds of Marines, was criticized by Newsom and local officials, who argued that the relatively small demonstrations could have been easily handled by city and state law enforcement.
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