The National Transportation Safety Board (NTSB) said it is investigating the crash of a Sikorsky S-61N helicopter on Feb. 22 that left four people dead. Four people were killed after the helicopter crashed during a training operation on the Hawaii island of Kauai, officials confirmed. The incident occurred at the Pacific Missile Range Facility (PMRF), which is the “world’s largest instrumented, multi-dimensional testing and training missile range,” according to the U.S. Navy. The Navy base is located in Kauai, Hawaii, and the crash occurred shortly after 10 a.m. local time on Tuesday, the Navy’s Pacific Missile Range Facility said. At the time of the incident, the helicopter—a Sikorsky S-61N—had been taking part in a range training operation on the base when it crashed on the northern side of the installation. The helicopter was operated by Croman Corporation, which “provides range support services” to the base, the facility said. According to its website, Croman is one of the world’s most experienced …