SAN DIEGO (AP) — Four Marines were killed Wednesday in a training accident at Camp Pendleton in Southern California, base officials said.

The accident happened at 11 a.m. during a range maintenance operation at the San Diego County coastal base. Officials were investigating the cause and provided no further details on the training or the accident.

The identities of the dead were being withheld pending notification of relatives.

“We offer our heartfelt prayers and condolences to the families of the Marines lost today in this tragic accident,” said Brig. Gen. John W. Bullard, commanding general of Marine Corps Installations West at Marine Corps Base Camp Pendleton. “Our first priority is to provide the families with the support they need during this difficult time.”

The deaths come about eight months after a mortar explosion killed seven Marines during a live-fire training exercise in Nevada.

A military investigation determined human error was to blame for that accident. According to the probe’s findings, a Marine operating a 60 mm mortar tube and ammunition did not follow correct procedures, resulting in the detonation of a high explosive round at the mortar position.

The investigation also determined that the mortar team involved in the accident had not conducted “appropriate preparatory training.”