U.S. Marine Corps AH-1Z Viper helicopters assigned to Marine Light Attack Helicopter Squadron (HMLA) 367 and CH-53E Super Stallion helicopters assigned to Marine Heavy Helicopter Squadron (HMH) 463 take off and land at Pacific Missile Range Facility Barking Sands, Hawaii, Nov. 2, 2021. The U.S. military said Tuesday it’s responding to “an aviation incident” on the island of Kauai in Hawaii. (Dalton Payne/U.S. Marine Corps)

Four civilians were killed Tuesday when a military-contracted helicopter crashed at the Pacific Missile Range Facility on Kauai Island, Hawaii, the Navy said in a news release.

The Sikorsky S-61N went down in the northern area of the installation around 10 a.m., the Navy said. The names of the civilians had not been released as of Tuesday evening.

The National Transportation Safety Board said in a tweet Tuesday that it was investigating the crash.

The S-61N, a civil version of the military’s Sea King medium-lift helicopter, is optimized for amphibious operations and can carry up to 15 passengers.

The helicopter, operated by Croman Corp., was supporting a training operation, the Navy said.

The helicopter had retrieved an object from the water and was attempting to drop it to the ground when it crashed, Brian Beattie, Croman director of operations, told Hawaii News Now.

The firm routinely provides support services to the facility.

The Navy touts Pacific Missile Range Facility Barking Sands as capable of supporting surface, subsurface, air and space operations simultaneously.

The facility includes more than 1,100 square miles of underwater range and over 42,000 square miles of controlled airspace.