Pilot error and other factors caused a nighttime collision of two helicopters that killed 12 Marines off the coast of Hawaii earlier this year, the Marine Corps said.

The disclosure on Tuesday stemmed from an investigation that also found low aircraft readiness that led to inadequate pilot proficiency, Marine Corps Forces Pacific spokesman Lt. Col. Curtis Hill said in a statement. The squadron's lack of focus on basic aviation practices also contributed to the January crash, he said.

The helicopters collided off Oahu's north shore. Hours later, rescue crews spotted debris 2 1/2 miles off Oahu and a five-day search followed.

Investigators believe poor light also made it difficult for the crews to gauge the distance between the two aircraft.

The III Marine Expeditionary Force, which included the helicopters, conducted the investigation. The Marine Corps plans to post the investigation report on its website.

The CH-53E helicopters were part of Marine Heavy Helicopter Squadron 463 based at a Marine base in Kaneohe Bay.

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.

In February, the Marine Corps said the commander of the 1st Marine Aircraft Wing had fired the squadron's commander on Jan. 11, just days before the crash.