Marines

Details on cause of deadly Marine chopper crash off Hawaii revealed

Marines searching for debris after the crash in January.

Marines searching for debris after the crash in January.  (Cpl. Ricky S. Gomez/U.S. Marine Corps via AP)

At least one of the pilots of two U.S. Marine helicopters that collided midair off Hawaii in January was not fully qualified to fly, an investigation revealed Wednesday.

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The crash near Oahu killed all 12 Marines on board. A search for survivors lasted days to no avail. A witness described seeing a fireball before the aircraft disappeared.

Also, the Marines confirmed the Pentagon's internal watchdog would review whether aviation teams are fully prepared to fly, a development first revealed by Inside Defense. The Defense Department's Inspector General notified the Marines last month, the Marine Corps Times reported.

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"Based on the trend analysis from June-July 2016, the Marine Corps' in-reporting and flyable aircraft have steadily increased since December 2014," Marine Capt. Sarah Burns told Fox News.

The key factors contributing to the January crash were "low aircraft readiness leading to inadequate pilot proficiency, human factors, and the squadron's lack of focus on basic aviation practices," Marine officials told Fox News.

The investigation found three of the four pilots had logged five hours or less of flight time in the 30 days before the crash. One pilot had not logged any hours of night flying time in the previous three months.

Marine officials also said all the pilots and crew "were qualified in accordance with regulations and standards and medically fit for duty."

The Marines were alerted when the CH-53E helicopters carrying six crew members each failed to return to their base at Kaneohe Bay following a nighttime training mission on Jan. 14. Hours later, a Coast Guard helicopter and C-130 airplane spotted debris 2 1/2 miles off of Oahu.

Marine officials said the pilots likely didn't realize how close they were because it was so dark outside.

The transport helicopters were part of the 1st Marine Aircraft Wing at Marine Corps Base Hawaii. Known as Super Stallions, they are the U.S. military’s largest helicopter, capable of carrying a light armored vehicle, 16 tons of cargo or a team of combat-equipped Marines, officials said.

Fox News' Lucas Tomlinson and The Associated Press contributed to this report.