The round was fired during a live-fire demonstration at Camp Pendleton held for the Marines' 250th anniversary.
According to reporting by the Los Angeles Times, investigators determined that the device's fuse activated prematurely, causing the round to detonate midair.
The explosion sent shrapnel onto the freeway, and one piece punctured a California Highway Patrol vehicle.
Marine officials described the incident as a "one-in-a-million" malfunction and said they still do not have a definitive explanation for what caused it.
Eyewitness News reached out to Gov. Gavin Newsom's office for comment. Chief Deputy Director of Communications Diana Crofts-Pelayo shared the following statement.
"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."