Four Marines were killed during a range maintenance operation Wednesday morning at Camp Pendleton in California, according to multiple reports.
Officials at the camp said the incident happened around 11 a.m. PT, according to the NBC affiliate in San Diego.
A military official said the incident involved the detonation of an unexploded ordnance, according to NBC News.
The cause of the incident, however, is under investigation.
In March, seven Marines from Camp Lejeune were killed and several were injured as a result of a mortar shell explosion during a training exercise in Nevada’s high desert.
Pentagon officials, at the time, halted the use of the weapons until an investigation could determine they were safe to use.
It’s unclear if the incident on Wednesday was similar to the one in March.