Teams searching the waters off Hawaii where two Marine helicopters crashed have had no luck so far in their hunt for 12 missing crew members.

Three of the four life rafts aboard the helicopters have been recovered and efforts were being made to retrieve the fourth. Coast Guard Chief Petty Officer Sara Mooers said Monday that some of the rafts were inflated, but it was unclear how they got that way.

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

Monday was the fourth day of searching. Conditions have improved since the start of the search, with much smaller waves.

Various agencies have been searching above water, below water and along the shoreline since the Coast Guard was notified late Thursday by a civilian who saw the aircraft flying and then saw 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 the crash and then dealing with possible dehydration, exposure and fatigue, said Mario Vittone, a retired Coast Guardsman who is an expert on sea survival. Vittone said survival seems unlikely, but he noted that he doesn't know all the circumstances and there's nothing unusual in the length of the search so far.

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.

A salvage ship from the Military Sealift Command arrived late Sunday from Pearl Harbor. It is supporting a Navy unit's efforts to perform an underwater search of the aircrafts' last known position with sonar and a remotely operated vehicle, the Coast Guard said in a news release. Searches by the unit on Sunday did not find any debris, the release said.