Twelve Hawaii Marines were killed Jan. 14 when two CH-53E Super Stallion helicopters collided off Oahu as one chopper raced at night to catch up and the first helicopter turned into its path, according to the Investigative Reporting Program at the University of California, Berkeley.

The impact resulted in an explosion that instantaneously killed all aboard, the university program said. Six Marines were in each aircraft during the night training.

According to the program’s report, Pegasus 32 outpaced the other helicopter, Pegasus 31, which accelerated just as the lead aircraft was making a sharp left turn, resulting in the collision at 1,500 feet elevation off Oahu’s North Shore.

The Kaneohe Bay pilots had fallen behind on flying hours, and two of them were not adequately proficient in the use of night-vision goggles, the Investigative Reporting Program said.

The California program worked with families of the fallen Marines and reported the findings Tuesday on Huffington Post and Civil Beat. The Marine Corps is expected to formally release its investigation today.

Lt. Col. Curtis L. Hill, a spokesman for Marine Corps Forces Pacific, said in an email late Tuesday that the Marines’ “investigation found the primary cause of this mishap to be pilot error.”

The inquiry “determined that the aircraft failed to maintain adequate distance during the flight,” and they collided, Hill said. “Investigators believe that the low light conditions made it difficult for the aircrew to recognize the rapid decrease in separation between the aircraft which led to the collision.”

The evidence indicated both aircraft were mechanically sound.

“Investigators found the main contributing factors were low aircraft readiness leading to inadequate pilot proficiency, human factors, and the squadron’s lack of focus on basic aviation practices,” Hill also said.

Killed in the crash were:

>> Maj. Shawn M. Campbell, 41, pilot.

>> Sgt. William J. Turner, 25, crew chief.

>> Capt. Kevin T. Roche, 30, pilot.

>> Capt. Brian T. Kennedy, 31, pilot.

>> Sgt. Jeffrey A. Sempler, 22, crew chief.

>> Sgt. Adam C. Schoeller, 25, crew chief.

>> Sgt. Dillon J. Semolina, 24, crew chief.

>> Cpl. Thomas J. Jardas, 22, crew chief.

>> Cpl. Christopher J. Orlando, 23, crew chief.

>> Capt. Steven R. Torbert, 29, pilot.

>> Cpl. Matthew R. Drown, 23, crew chief.

>> Lance Cpl. Ty L. Hart, 21, crew chief.

“This loss of life was tragic and is felt deeply in the Marine Corps community,” Hill said.“Our thoughts go out to the families of all those affected by this incident.”

After the crash, the Marine Corps said it grounded its CH-53E Super Stallion helicopters from training in Hawaii for 19 days and pulled them from an Australia deployment.

The collision in a fireball 2 miles off the coast came amid revelations that Marine Corps aviation — including fixed-wing F/A-18 Hornets as well as rotary aircraft — had been hamstrung by budget cuts leading to parts shortages and maintenance backlogs.

Lisa and Mike De La Cruz, Semolina’s parents, previously told the Honolulu Star-Advertiser that the two choppers shouldn’t have been flying that night because the parts issue was so bad.

Marine Heavy Helicopter Squadron 463, which had 12 choppers before the crash, “stopped all flight operations for 12 days and then only conducted functional check-flights required in support of maintenance-related operations,” Capt. Cassandra Gesecki, then a spokeswoman for the III Marine Expeditionary Force in Okinawa, Japan, said in April. “There were no training or operational support flights flown for a total of 19 days.”

Three days before the crash, Lt. Col. Edward Pavelka, commander of HMH-463, was relieved of command.

Higher command had “lost confidence in his ability to continue to lead the squadron,” the Marine Corps said without explaining what Pavelka had done.

An inquiry made by Minnesota U.S. Sen. Al Franken’s office said Pavelka was removed because the squadron had low unit morale and low readiness.

Mike De La Cruz previously said he was told Pavelka was flying the helicopters minimally because of safety concerns.

By August the Marine Corps had begun a full refurbishment of its CH-53E Super Stallion heavy lift helicopters, “an effort aimed at significantly increasing the number of operationally fit aircraft and addressing systemic issues, which in recent years drove the platform’s readiness level to unsustainable depths,” Naval Air Systems Command said at the time.

Officials found the material condition of the CH-53 helicopters degraded due to age and hard use in war in Iraq and Afghanistan.