Two Marine Corps pilots died in a helicopter crash during a training mission near Yuma, Ariz., over the weekend.

Marine Corps spokesman Capt. Gabriel Adibe said that the crash happened Sunday night on the Marine Corps Air Station Yuma training ground, but the cause of the crash is still under investigation, according to the Associated Press.

The pilots were conducting training in a AH-1Z Viper attack helicopter as part of a weapons and tactical instructor course at the time of the crash.

No additional information about the fatal crash was immediately available and the names of the pilots have not yet been released.

The training ground where the crash took place, in the southwestern part of Arizona, is one of the world’s largest military installations, at 1,300 square miles.

According to a 2018 report from Military Times, Marine Corp aviation incidents have risen by 80 percent in the last five years, something that the report correlates to automatic budget cuts, known as sequestration, which took effect the year incidents began to rise.