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A fuze malfunction, the second ever in the model’s service history, likely caused a M795 155mm artillery round to explode early in a live-fire demonstration at Camp Pendleton last fall, raining shrapnel onto a civilian highway, the Marine Corps found.

The Marine Corps released its own internal investigation into the Oct. 18 incident on Friday. The incident last fall saw an artillery shell prematurely detonate over Interstate 5 while Marines carried out a live-fire barrage as part of an amphibious assault demonstration. The event, held as part of celebrations for the Marine Corps’ 250th birthday, was attended by Vice President J.D. Vance and Defense Secretary Pete Hegseth, and caused confusion in the hours leading up to it, prompting a last-minute closure of Interstate 5 near the base. In the hours ahead of the celebration, electronic signs set up along the highway dozens of miles from the base warned drivers of “Live weapons over freeway” as they approached Camp Pendleton.

The Marine Corps determined the issue was the round’s electronic fuze malfunctioning, although the cause of the malfunction wasn’t clear. Per the report, prepared for the commanding general of I Marine Expeditionary Force, the fuze was “manufactured to a one-in-a-million defect standard.” Six M777 howitzers were set up on Red Beach and crewed by Marines from 2nd Battalion, 11th Marine Regiment, 1st Marine Division. They were set to fire artillery into a training area inside Camp Pendleton, on the other side of the freeway. 120 artillery shells were available for the demonstration, the Marine Corps said, and the rounds were fired over Interstate 5, one of California’s busiest highways and serves as the main route between San Diego and Los Angeles; Camp Pendleton sits between the two cities. An Amtrak train line also runs parallel to the freeway by the base.

Investigators couldn’t determine what caused the malfunction, but said that the howitzers’ proximity to one another might have contributed. The six guns fired were closely packed together, placed in a space 165 yards by 165 yards, with two howitzers only 14 feet apart. The report said that “several career artillery officers note they have never seen howitzers placed this close together but also acknowledged that there was no restrictive minimum distance of Gun placement in relation to administrative artillery live fire.” Additionally, the “potential presence of anomalous electromagnetic energy in the vicinity” of the artillery pieces might have played a role, although the report did not elaborate on what that meant.

“The M767A1 fuze is extremely reliable, with only one malfunction in its service history,” the report said. That incident happened at Fort Drum in 2017 when the same fuze, mounted to a 105mm M119A3 round, exploded early after hitting something mid-flight.

Experts, including engineers at the Naval Surface Warfare Center, Corona in California examined the evidence from the events leading up to the incident. The shell detonated 453 meters above the highway. Shrapnel, including a two-inch-long piece, fell to the ground, hitting a California Highway Patrol car and motorcycle. No one was injured. The two vehicles hit by the raining shrapnel happened to be assigned to Vance’s security detail; Vance himself was not nearby, instead at the stage where the event’s speakers were at.

The report also noted that President Donald Trump was originally set to attend the amphibious landing demonstration, which would have necessitated the closure of the highway by Camp Pendleton. However, Vance, a Marine veteran, went instead, which created a possibility of traffic being allowed to pass through.

The premature detonation was the culmination of a chaotic period, after the Marine Corps initially determined that no closure was needed following its own risk assessment. The California government decided, after back and forth with the military and federal government, to close 17 miles of the freeway near the base for several hours, out of safety concerns. Those included the direct risk of the artillery and potential risk the sound of the artillery could cause to drivers, startling or distracting them.

It was later revealed that Marines fired 30 155mm artillery rounds over Interstate 5 the day prior during a rehearsal, while the highway was still open and cars passed under the arc of the munitions. In the fall, California Highway Patrol said that the Marine Corps did not specify the rehearsal would include live artillery rounds being fired, although the Marine Corps claimed that was announced to California officials.

LOS ANGELES – The detonation of a military round over Interstate 5 during an exercise at Camp Pendleton in October was a “one in a million” malfunction that investigators have struggled to explain, according to a report from the U.S. Marine Corps released Friday.

The investigation determined the military round exploded before it was supposed to because the device’s fuse went off early, raining metal shrapnel below. A California Highway Patrol cruiser was punctured by metal.

“Of all days for this very low probability event to happen, why this one? What was different from the thousands of times before this event employing the same shell-fuse combination, weapons system, and highly trained Marines?” the report read. “There is no definitive answer to these questions.”

The incident occurred Oct. 18 during the Marine Corps’ 250th anniversary celebration, after Vice President JD Vance had been escorted to Camp Pendleton to witness the exercise. A 17-mile stretch of the interstate was stopped half an hour before the exercise was set to begin at 1:46 p.m., drawing the ire of thousands of weekend travelers.

The White House initially criticized California Gov. Gavin Newsom for shutting down the freeway during the military exercise, stating on X that “the Marines repeatedly said there are no public safety concerns.”

But after word spread of the early detonation, Newsom called on the Trump administration to apologize, saying the exercise was “reckless” and one of the White House’s “vanity projects.”

“We’re thankful to the Marines for their thorough and precise investigation – in stark contrast to the dangerous and performative demands by JD Vance and Pete Hegseth to shoot live ammunition over a civilian area for their entertainment,” said Diana Crofts-Pelayo, a spokesperson for Newsom’s office in a statement.

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

According to the report, there were no injuries from the falling shrapnel, but it did strike the CHP vehicle as well as a CHP motorcycle near Las Pulgas Road.

The report suggested possible causes for the premature fuse detonation but determined that military personnel were not at fault.

The shrapnel came from an M795 round that had been paired with an M767A1 fuse, which was meant to electronically detonate the round at a predetermined point, the report said.

Instead, the round detonated prematurely at 453 meters, or about 1,486 feet, in the air, around 3,300 feet from its firing location, according to the report.

The size of the shrapnel that struck the CHP vehicles was described as 2 inches by 2 1/2 inches.

The report states that the fuse that was used in the exercise is “extremely reliable” and had only one previous recorded malfunction in 2017.

“It is manufactured to a tolerance of one defect in a million,” the report states. “The likelihood that the M767A1 fuze malfunctioned is a statistical anomaly that was outside any reasonable probability.”

Investigators ruled out the possibility of the round striking a drone in the area and, although it was possible it might have struck a bird, noted that “none were observed along the flight path of the projectiles during the incident, and no organic residue was found on shell fragmentation.”

Instead, the report noted that the M777 Howitzers, the artillery used to fire the rounds, might have been too close to one another as they fired, or the fuse might have been exposed to “anomalous electromagnetic energy in the vicinity.”

According to the report, the positions of the Howitzers were changed three days before the event, occupying a 50-by-50-meter box that was smaller than originally planned. That meant that some guns were 10 to 15 feet apart, according to the report.

“Several career artillery officers note they have never seen howitzers placed this close together but also acknowledged that there was no restrictive minimum distance of Gun placement in relation to administrative artillery live fire,” the report reads.

The report states that investigators 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 such measures.

According to the report, President Trump had been scheduled to attend the event initially but Vance was instead sent in his place.

An artillery shell that exploded over Interstate 5 near Camp Pendleton during an October live-fire demonstration, scattering shrapnel onto the closed freeway, was caused by a rare malfunction, according to a U.S. Marine Corps investigation.

Shrapnel from the incident landed on a California Highway Patrol vehicle and motorcycle that were part of the security detail for Vice President JD Vance, who attended the event with Secretary of War Pete Hegseth in honor of the U.S. Marine Corps' 250th birthday celebration.

The 666-page report was released Friday by the commanding general of I Marine Expeditionary Force at Camp Pendleton. The incident occurred Oct. 18 during a large-scale military demonstration on the base. Marines fired a 155- millimeter artillery round from a howitzer toward a designated training impact area when the projectile detonated prematurely in midair.

Investigators determined the likely cause was a malfunction in the shell's electronic time fuze, a component designed to control the timing of the explosion. The failure was described in the report as extremely rare, with officials saying the device has a very low historical malfunction rate.

The round exploded about 1,500 feet above ground, sending fragments downward onto a closed section of I-5 that runs along the western edge of the base. No one was injured and officials halted the exercise immediately after the explosion.

Dozens of artillery rounds had been scheduled for the demonstration, which was part of a broader training event and public showcase of Marine capabilities.

The investigation found no evidence of operator error and concluded that Marines followed approved safety procedures. The report noted that factors such as the proximity of multiple artillery guns firing in quick succession or possible electromagnetic interference could have contributed to the malfunction, though investigators could not determine a definitive secondary cause.

The event drew criticism from some California officials who questioned the decision to conduct live-fire artillery training near a major civilian transportation corridor.

Marine Corps officials said the demonstration took place within an approved training area and that safety protocols including the freeway closure functioned as intended.

The USMC said it is reviewing the findings from the closed investigation and may adjust procedures for future demonstrations conducted near public infrastructure.