Crews searching for 12 Marines who were in two helicopters that went down off Hawaii earlier this week have recovered debris that is "consistent" with the missing aircraft, a Coast Guard official said Sunday.

Capt. James Jenkins, chief of staff for the 14th Coast Guard district in Honolulu, did not specify what had been recovered. KHON reported that the debris was found off the North Shore of the island of Oahu.

Rescuers from various agencies have been searching round-the-clock since the Coast Guard was notified late Thursday of the crash by a civilian who saw the aircraft flying and then disappear and a fireball.

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.

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, according to a Marine Corps website.

The wing's commanding general, Brig. Gen. Russell Sanborn, told reporters Sunday he has personal experience with the "emotional roller coaster" families of the 12 Marines are experiencing.

His wife went through similar emotions when he was shot down 25 years ago during Operation Desert Storm and was listed as missing in action, he said.

The Coast Guard initially reported that the choppers had collided, but Marine Capt. Timothy Irish said Friday that he did not know if the accident was a collision.

"We think they collided because both of them went down," Mario Vittone, a retired Coast Guardsman who is an expert on sea survival, said Sunday from Florida.

It would have happened very quickly and survival would entail overcoming many factors. "You have to survive the crash, you have to survive the on-rushing of water," he said, adding they would then have to battle dehydration and exposure to the elements.

Meanwhile, high surf that has complicated the search for the missing men was expected to subside to 10 to 15 feet on Monday and decrease to 8 to 12 feet along west-facing shores.

It's difficult to spot anything in breaking waves, said Vittone, who was a helicopter swimmer and marine accident investigator with the Coast Guard. "It almost like camouflages everything."

The crash was off the north shore, but the search area was expanded to include waters off Oahu's west coast on Saturday.

"Before we stop, we will notify all of the family members so they will have advance notice that we intend to stop," Capt. James Jenkins, chief of staff for the 14th Coast Guard district in Honolulu, told reporters.

In addition to Coast Guard assets, the search effort includes Navy ships and aircraft, an Army helicopter, the National Guard, along with Honolulu's fire, police and ocean safety personnel. Marine Corps members were combing the beach for debris.

Some family members were holding out hope that survivors could be found, while asking for privacy as they waited for updates.

"My husband and I want everyone to know that this is not about us," Donna McGrew, mother of Maj. Shawn Campbell of College Station, Texas, said in a statement. "This is about the families that are suffering, and about all the sacrifices that our military members and their families make on a daily basis."

The missing crew members are:

— Maj. Shawn M. Campbell, 41, College Station, Texas.

— Capt. Brian T. Kennedy, 31, Philadelphia.

— Capt. Kevin T. Roche, 30, St. Louis.

— Capt. Steven R. Torbert, 29, Florence, Alabama.

— Sgt. Dillon J. Semolina, 24, Chaska, Minnesota.

— Sgt. Adam C. Schoeller, 25, Gardners, Pennsylvania.

— Sgt. Jeffrey A. Sempler, 22, Woodruff, South Carolina.

— Sgt. William J. Turner, 25, Florala, Alabama.

— Cpl. Matthew R. Drown, 23, Spring, Texas.

— Cpl. Thomas J. Jardas, 22, Fort Myers, Florida.

— Cpl. Christopher J. Orlando, 23, Hingham, Massachusetts.

— Lance Cpl. Ty L. Hart, 21, Aumsville, Oregon.

The Associated Press contributed to this report.

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