How 2 elite Navy SEALs drowned in plain sight in anti-terror mission

Two U.S. Navy SEALs drowned during a nighttime boat raid off the coast of Somalia last January because their personal gear was too heavy, causing them to sink almost immediately upon hitting the water, according to the results of a military investigation released Friday.

The findings by the Naval Special Warfare Command mark a tragic final chapter in a nine-month quest to understand how two highly trained elite military operators -- including a Division I college swimmer -- drowned and their bodies lost to sea as helicopters and drones circled above.

The two SEALs were identified as Nathan Gage Ingram, 27, who was posthumously promoted to Special Warfare Operator 1st Class, and Christopher Chambers, who was posthumously promoted to the rank of Chief Special Warfare Operator.

Chambers, the champion swimmer, died just days ahead of his 37th birthday. Ingram, who had tried to save Chambers, was on his first deployment.

"This incident, marked by systemic failures, was preventable," wrote Gen. Michael "Erik" Kurilla, head of U.S. Central Command, the combatant command that oversees missions in the Middle East.

In a statement exclusive to ABC News, the Ingram family thanked the Naval Special Warfare Team for their "efforts and diligence" throughout the investigation and said they were honored by his posthumous promotion.

"While we miss him dearly, we are comforted by the thoughts and prayers of friends and family, both near and far," they wrote.

"We remain immeasurably proud of his heroic sacrifice in service of this Country. We carry him in our hearts each day, remembering always his devotion to his Country, his brothers, and his family," the statement added.

'Just 47 seconds'

On Jan. 11, a slow-moving cargo boat called a dhow was making its way across the Arabian Sea off the coast of Somalia.

Officials told investigators they believed the dhow was stocked with Iranian-made weapons -- ballistic and cruise missile parts -- that were being shipped to Houthi militants, who for months had been attacking commercial and military vessels in the region as a kind of protest against the war in Gaza.

The military plan called for sending a nine-person team -- including SEALs Chambers and Ingram -- to board the smuggling boat that night. According to the investigation, commanders wanted to give the personnel time to prepare for the mission while still being able to pull the dhow while it was still transiting international waters.

With two helicopters circling 200 feet overhead and drones hovering, the team departed from the USS Lewis B. Puller aboard three specialized speedboats. Pulling up alongside the smuggling boat, a light tactical ladder padded with grip tape was thrown over the ship's railing, with one member declaring a "perfect hook," according to the report.

Some team members opted to ignore the ladder and climbed directly over the dhow's railing, later telling investigators they thought the railing had been slippery and feeling as though it had just been painted.

While the report redacts the names of the SEALs, the details were confirmed by people who were briefed on the unredacted version and other publicly available accounts of events.

The last two members set to board were Ingram and Chambers, the latter a high school state champion who swam for the University of Massachusetts Amherst and the University of Maryland.

According to the investigation, Chambers opted to jump and grab the railing wearing as much as 48 pounds in gear. With waves now rocking hard against the stalled vessel and the weight of his gear pulling on him, Chambers quickly lost his grip and plunged into the water 9 feet below.

While he resurfaced briefly and grabbed part of the ladder attached to the dhow, aerial imagery reviewed by investigators suggested Chambers was "swept under by a wave," according to the report.

Watching from above was Ingram, the younger SEAL on his first deployment.

"Observing his teammate struggling," Ingram "jumped into the water to render [Chambers] assistance," according to the report.

Ingram was estimated to be wearing as much as 80 pounds in gear, with Navy investigators believing he was carrying a radio rucksack for the team. He, too, quickly sunk.

With "his helmet barely breaking the surface," Ingram appeared to have deployed a special floatation device from his kit and at one point tried to shed his gear, according to the report. But for reasons not entirely clear, the floatation device was later found intact drifting in the ocean, the Navy found.

"Encumbered by the weight of each individual's gear, neither their physical capability nor emergency supplemental flotation devices, if activated, were sufficient to keep them at the surface," the Navy investigation concluded.

"The entire tragic event elapsed in just forty-seven (47) seconds, and two NSW warriors were lost to the sea," according to the probe, using the acronym for Naval Special Warfare.

Man overboard

While "man overboard" procedures were quickly put in place, the report suggested Chambers and Ingram were surrounded by colleagues who assumed they couldn't sink.

"The prevailing expectation within the special operator community -- expressed by [a] number [of] interviews ... was that in the event of a man overboard, the operator would return to the surface to effect a rescue. This was noted in at least 12 interviews," the report states.

According to the Navy, SEALs are warned to test their "buoyancy" ahead of a deployment -- their ability to float while carrying their gear -- but there's no specific guidance on what that means. And it's mostly up to individual SEALs on to how to gear up for a mission, meaning no one was required to check Chambers, Ingram or the others in advance to ensure they could still tread water if they fell into the ocean.

Another factor, according to the investigation, was that there was little to no training among the special operators on how to trigger the tactical flotation device in an emergency. SEALs interviewed by the investigators said they used the device -- a kind of specialized "water wings" for operators -- once, if at all, in their careers, according to the report.

Investigators ultimately dismissed other factors -- including the accelerated timetable for the mission -- as contributing to the incident.

"The root cause of these drownings was a lack of a fail-safe and layered defense to provide buoyance ... whether to overcome the weight of equipment, fatigue from extreme physical exertion and resultant inability to tread water, or an injury suffered during the boarding process," the Navy concluded.

The Navy searched for Chambers and Ingram for 10 days, believing it was possible their remains could still surface. But after scanning nearly 49,000 square nautical miles, the Navy declared Ingram and Chambers presumed dead.

Officials now believe it's likely the two never drifted, sinking "straight down due to weight," according to the investigation.

Looking ahead, the Navy is recommending changes to training and considering new standards for gear checks in advance of a mission. It also recognizes Ingram as a hero.

"In his effort to provide rescue and assistance to his teammate, he ultimately gave his own life, demonstrating heroism and bearing witness to the best of the SEAL Ethos," the Navy wrote in its report.

ABC News' Nate Luna contributed to this story.