Incident causes 155 mm artillery shell to explode over California highway

The US has released investigation results, revealing an M795 artillery shell exploded and scattered fragments onto a California highway due to a rare incident during a military celebration.

The Associated Press (AP) on 18/3 published the results of a US Marine Corps investigation into an M795 artillery shell explosion directly above Interstate 5 (I-5) in California. The incident sent fragments onto the highway, striking two state highway patrol vehicles.

According to the investigation, the incident occurred on 18/10/2025, during Camp Pendleton's celebration of the 250th anniversary of the US Marine Corps. Vice President JD Vance and Secretary of Defense Pete Hegseth attended the event. Organizers planned to fire 60 M795 artillery shells using six M777 howitzers over five minutes. These were live 155 mm rounds, not ceremonial blanks.

The US fires a howitzer during a demonstration exercise at Red Beach, Camp Pendleton base, California, on 19/10/2025. Photo: Reuters

A day before the event, California Governor Gavin Newsom, a Democrat, deemed Camp Pendleton's live-fire plan "dangerous and unnecessary". He ordered the I-5 highway closed during the ceremony. The White House and many Republican politicians criticized the decision to close I-5. However, observers believe it helped minimize casualties when the incident occurred.

The organizers calculated the artillery shells would fly over the highway and detonate in the hilly area on the opposite side. The US Marine Corps stated the demonstration would not endanger commuters. However, during the first volley, one shell prematurely exploded at an altitude of 450 m, scattering fragments onto the vital highway connecting Los Angeles and San Diego. All subsequent firings were canceled.

Investigators concluded there were no signs of negligence or misconduct from the soldiers, and "no definitive answer" regarding the shell's explosion. The report stated such a premature explosion was "beyond all expectations, should not have occurred, but ultimately did". This type of ammunition is manufactured with a tolerance of "one defect per one million products".

Location where artillery shell fragments fell on I-5 highway in California. Graphic: ABC7

Days after the incident, California officials sent a letter to Mr. Hegseth, questioning who approved the live-fire plan over the highway. They also requested clarification on how organizers prepared for associated safety risks. The US Marine Corps report suggested the incident could stem from multiple factors, including the howitzers being positioned too close together during firing, and the "potential for unusual electromagnetic energy in the area".

"What was different compared to thousands of previous instances, using the same weapon system and well-trained personnel? There is no definitive answer", the report concluded.