Authorities searching the area where two Marine helicopters crashed off Hawaii have found some life rafts that were carried aboard the aircraft, but still no sign of the 12 crew members who were on board.

Coast Guard Chief Petty Officer Sara Mooers said Monday she believes three life rafts have been recovered so far. Some were inflated, but it was unclear how they came to be inflated, she said.

There is no indication that anyone was aboard the rafts, based on their condition and the lack of any personal effects, she said.

The search for the Marines entered its fourth day Monday, with plans to search into the night. Conditions have improved since the start of the search, with much smaller swells expected Monday.

Rescuers from various agencies have been searching 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. Hours later, a Coast Guard helicopter and C-130 airplane spotted debris 2 1/2 miles off of Oahu.

The crash was near the north shore, but the search area spans from the western coast of Oahu to the northeast corner of the island. At this point, it's still a search for survivors, Mooers said.

The Coast Guard assumes the best-case scenario when considering how long someone in the right equipment and right conditions could survive, she said.

"We err on the side of caution because the last thing that anybody wants is to suspend the search when there's still a possibility of finding somebody," she said.

Marine Capt. Timothy Irish said Monday that aircrews wear personal flotation devices with their flight suits and get additional training on top of survival swimming training. There are various ways that life rafts could be inflated, including a cord being pulled by debris, he said.

Mooers said people have been founds days or even weeks after they've been at sea.

Survival would entail overcoming many factors, including surviving the crash, being overwhelmed by water, and then facing dehydration and exposure, said Mario Vittone, a retired Coast Guardsman who is an expert on sea survival.

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.

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 Coast Guard initially reported that the choppers had collided, but Irish said Friday that he did not know if the accident was a collision. The cause remained under investigation.

Mooers said the helicopters carry flight data recorders known as black boxes, but they have not been recovered.

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.