Four people are dead after a civilian-contracted helicopter crashed at a U.S. naval facility, officials said.

The incident occurred at the Pacific Missile Range Facility, a Navy base in Kauai, Hawaii, shortly after 10 a.m. local time Tuesday.

The helicopter was operated by Croman Corporation, which “provides range support services” to the base, a Pacific Missile Range Facility spokesperson said in a statement.

The helicopter was taking part in a range training operation on the base in Kekaha when it crashed on the northern area of the installation, the spokesperson said.

Emergency personnel responded to the crash, the spokesperson said. There were four fatalities. The names of those killed have not yet been released.

All four crew members were Croman employees, Brian Beattie, the company’s director of operations, said in a statement.

“The helicopter was conducting routine training operations under contract to the U.S. Navy,” Beattie said. “No further details are available.”

An investigation is underway to determine the cause of the accident, the Pacific Missile Range Facility spokesperson said.

The National Transportation Safety Board also confirmed it is investigating the crash of a Sikorsky S-61N helicopter near Kekaha.

The Federal Aviation Administration is also investigating, Beattie said.

Pacific Missile Range Facility is the “world’s largest instrumented, multi-dimensional testing and training missile range,” according to the U.S. Navy. It has over 42,000 square miles of controlled airspace.