Rescuers launched an underwater search Sunday for any clues to the whereabouts of 12 Marines who have been missing since the two helicopters they were in collided off Oahu on Thursday night.

Coast Guard spokesman Lt. Scott Carr said a Navy diving salvage unit arrived Sunday and deployed a remotely operated vehicle but that no significant discoveries were made. It was the first time rescuers have been able to search underwater.

Carr said the unit became available Sunday, and sea conditions for the dive team were the best since the search mission began. Swells were about 10 feet Sunday, down from about 16 feet on Friday and Saturday.

A Navy ship equipped with sonar was also expected to arrive Sunday and begin conducting an underwater search today of the last known position of the helicopters, about 2 miles off Haleiwa.

Carr said the Coast Guard planned to continue searching overnight Sunday with a patrol boat, an HC-130 Hercules airplane and an MH-65 Dolphin helicopter.

The Coast Guard, which is leading the search mission, has been continuously scouring the ocean by sea and air since responding to the crash of two CH-53E Super Stallion helicopters on a routine training mission late Thursday night.

By Sunday evening the Coast Guard completed 78 individual searches and covered more than 18,000 square miles, primarily between Kaena Point and Kahuku.

“We’re dedicated to trying to locate and bring back these service members,” said Coast Guard Capt. Jim Jenkins at a news conference Sunday afternoon outside the command post at Haleiwa Alii Beach Park, which was closed all weekend due to the massive multiagency operation.

The search effort Sunday included Navy ships and aircraft, an Army helicopter, a Coast Guard patrol boat and helicopter, lifeguards, police and firefighters, Jenkins said.

Marine spokesman Capt. Timothy Irish said about 45 Marines were walking the shoreline Sunday, looking for debris.

Lifeguards on personal watercraft also searched 160 linear miles on a search pattern tracked by GPS betwen Kahuku and Haleiwa by Sunday, said Shayne Enright, Honolulu Emergency Services Department spokeswoman. She said lifeguards would continue searching with two personal watercraft, and an ambulance was on standby at the beach park.

Brig. Gen. Russell Sanborn, commanding general of the 1st Marine Aircraft Wing, who flew in to Honolulu from Okinawa, Japan, on Sunday to assist in the search, said the top priority was taking care of the families of the missing.

“Our thoughts and our prayers are with those Marine families out there,” Sanborn said. “The Marine Corps, and me personally, are here for them and to support them in any way we can.”

He also thanked local first responders for their assistance.

Sanborn said he and his wife understand the emotional roller coaster the families may be going through, having experienced similar emotions when he was shot down during Operation Desert Storm about 25 years ago and was missing for days.

“We just want to let know that we’re here,” he said. “We’re here to put our arms around them and hug them, and let them know we love them. We’re going to do everything we can to support them.”

Jenkins said the Coast Guard is using computer models and deploying drift buoys to determine drift patterns and lay down search patterns. He said some debris that appeared to be from military aircraft was collected and turned over to the Marines who are investigating the cause of the crash.

The Marines said the helicopters did not have beacons similar to those on commercial airliners.

Jenkins said the Coast Guard has a protocol for how long to search for survivors and will notify family members before ending the search.

“Every day, we analyze what we’ve done; we analyze the likelihood of any survivors,” he said. “We’ll continue our efforts as long as needed.”