Officials say that 22 Marines were exposed to halon Thursday afternoon after a fire extinguisher system discharged without warning aboard an amphibious assault vehicle at a base in California.

The Marines were treated after an equipment malfunction caused the fire suppression system to erupt inside the vehicle during a training exercise at Marine Corps Air Ground Combat Center Twentynine Palms, according to a Fox News report.

Although there wasn’t any explosion, the incident required 22 Marines to be sent to a base hospital, and all but three of them were soon released to resume training. The remaining three had to stay for observation.

The Marines are part of the 2nd Battalion, 3rd Marine Regiment based in Hawaii. They were participating in an exercise at the base as part of integrated training.

Twentynine Palms is a base located 130 miles east of the city of Los Angeles and is the largest Marine training base on the entire planet, with thousands of Marines participating in live-fire drills. A total of 90 percent of Marines cycle through the base for combined-arms training in the desert base, according to the report.

The most recent fatal incident was when two Marine pilots were killed when their helicopter crashed in January during a training exercise, which is still under investigation.

Twentynine Palms is located in the San Bernadino County in California, and is part of the Mojave Desert. It is named for the plam trees that were found there by a surveyor in 1852. The Marine Corps base there was founded in 1952.