Military and civilian agencies entered their second day of battling high seas north of Oahu today as they continue to search for survivors of Thursday’s late-night crash of two Marine helicopters with a total of 12 Marines aboard.

Two CH-53 Super Stallion helicopters, on a “routine training”mission and each carrying six Marines, may have collided before dropping into choppy waters about 10:45 p.m. Witnesses described hearing a loud crash and explosions. There have been no sign of survivors and the Marines are investigating the cause of the crash.

The Coast Guard did not receive a May Day call, said Coast Guard Lt. Scott Carr at a press conference Friday afternoon at Marine Corps Base Hawaii at Kaneohe Bay, where the helicopters were based. “The Coast Guard was notified by a civilian on the beach who had seen the aircraft flying and then saw them disappear … (saw) a fireball, and then notified the Coast Guard,” Carr said. The call came at 11:38 p.m., he said.

Debris from the crash was sighted 2.5 miles off Haleiwa shortly after midnight, Coast Guard officials said. A Coast Guard spokeswoman told CNN that search crews spotted an empty life raft and flames on the water.

Crews were searching a debris field spread out over about seven miles offshore from Mokuleia Beach to Turtle Bay, according to the Coast Guard. The water depth varies from 156 feet to 1,400 feet between the half-mile and 7-mile marks in the area.

Capt. Tim Irish, Marine Corp Base Hawaii spokesman, said the Thursday night flights were “part of routine training,” and that the helicopters were scheduled to return to the base. He said that the two helicopters didn’t check in on time and were reported missing. “So we started working with the Coast Guard as the first responders.”

“It was aircrew, there were no passengers on board,” Irish said. He said the crew is normally four. “I have heard that there were one or two additional instructor-trainers aboard,” he said.

“This is still an active search-and-rescue operation” Irish said. “There are several Marine Corp. families affected right now and they’ve got a lot of concern for their loved ones and our hearts and prayers definitely go out to them as well as part of our family.”

The family of Capt. Kevin Roche say he was one of the Marines aboard the helicopters. “We believe the Marines and Coast Guard are doing everything they can to bring Kevin and his fellow Marines home safely, and we are grateful to everyone involved in the rescue,” said a family statement distributed by brother-in-law Anthony Kuenzel in St. Louis.

An aunt posted on a Marine Corps Facebook page that three Marines visited Roche’s parents and reported that he was missing.

The Coast Guard has established a safety zone from Kaena Point to Kahuku Point from the shoreline to 8 miles off shore.

“We have found debris across that entire area,” Carr said. “We have seen debris, haven’t recovered any of it. But the debris is consistent with military aircraft.”

“The Coast Guard is going to continue to search. Our goal is to find survivors,” he said.

Irish said 30 Marines from Marine Corps Base Hawaii are on the scene assisting with the search efforts. A Marine MH-60 helicopter crew from the 37th Helicopter Squadron out of Kaneohe is assisting in the search.

“And right now, from the base,” Irish added, “we’re pushing some additional resources out there.”

Items such as food and water for first responders as well as barrels to collect crash debris for analysis were being sent with the Marines, Irish said.

Joining in the search were two Pearl Harbor-based destroyers — the USS John Paul Jones and USS Gridley — and a Navy helicopter from Kaneohe Bay.

Marine Corps officials verified that the crash involved two helicopters from the Marine Heavy Helicopter Squadron 463 of the 1st Marine Aircraft Wing out of Kaneohe Bay. A Coast Guard spokeswoman told the Associated Press early Friday that the two helicopters collided.

The Marines are investigating the cause of the crash.

An HFD spokesman said at least six of its units and its helicopter and rescue boat are searching. HFD crews set up their command at Haleiwa Small Boat Harbor.

The Coast Guard said a MH-65 Dolphin helicopter and HC-130 Hercules aircraft were dispatched from Barbers Point and arrived on scene just after midnight and spotted the debris field.

Two Coast Guard rescue boats — the 110-foot patrol boat Kiska from Hilo and the 87-foot patrol boat Ahi from Maui — joined the search, as did a Navy MH-60 helicopter from Kaneohe Bay.

Many North Shore residents reported hearing loud explosions late last night and HFD said it dispatched firefighters at 10:52 p.m.

Haleiwa resident Mark Waugh, 42, said he was going to bed “about 11ish” when he heard the sound of helicopters when he heard “a big boom. It was very loud.” He said two friends who live on the shoreline also heard the crash.

Waugh, a North Shore surfer, plumber and former Army specialist, said he both knows the sound of helicopters and he knows the surf. He called Friday’s wave conditions “extreme monster surf.”

“God bless the troops,” Waugh added.

Shanee Giltner-Baptiste, 45, who lives in a home across from Haleiwa Alii Beach Park, said she heard the sound of helicopters at about 10:40 p.m. and then two loud explosions. “I thought it was a Schofield bombing; it was so strong,” she said.

The “booms” rattled her home twice, she said. Within moments she heard emergency responders and could hear talk on their radio chatter about 12 people on board.

Burt Sutherland, 59, of Pupukea, said he was using his computer shortly before 11 p.m. when he heard “two smacking, kind of banging-smacking sounds.” He too thought is was artillery from Schofield Barracks.

He said it was so loud he went up to his lanai to investigate. “There was no moon, no light. It was misty and raining,” Sutherland said.

He said military helicopters had been flying over the area for much of Thursday afternoon at an elevation that he estimated to be about 1,000 feet.

Ocean conditions are rough with ocean swells running about 20 feet and a high surf warning through 6 p.m. today for the north shores of Oahu, Kauai, Maui and Molokai.

Carr said because of the extremely dangerous high surf conditions on the North Shore, both surfers and beachgoers should exercise caution in the area.

He said the debris could potentially cause serious harm. “Even if anyone sees debris in the water,” Carr said, “they should not try to retrieve, but call the Coast Guard.”

Police have asked the public not to touch any debris that may be found on the shore between Haleiwa to Kahuku. People are advised to instead call the Coast Guard at (808) 535-3333, according to Irish.

After daybreak, dozens of people had come to Haleiwa Beach Alii Park to watch the search and rescue efforts. Shortly after 8 a.m., Honolulu police began clearing the beach and the park, telling onlookers that heavy military vehicles were on the way. City officials said the park could be closed for several days as the search continues.

The 1st Marine Aircraft Wing at Kaneohe Bay has been led by Brig Gen. Russell Sanborn since August. Helicopters assigned to the wing include CH-53 Super Stallions, AH-1 Super Cobras, UH-1 Hueys and eventually MV-22 Ospreys. A video of the squadron training was posted on the Marine Corps Base Hawaii Facebook page on Thursday.

This is the second military aircraft crash in less than a year and the second military helicopter crash in four years in Hawaii. On May 17, two Marines were killed and several others injured in a crash landing of an Osprey at Bellows Air Force Station. The tilt-rotor aircraft will be relocated to Kaneohe beginning this year with eventually 24 Ospreys based in Hawaii.

On March 29, 2011, Cpl. Jonathan D. Faircloth, 22, who had survived combat tours in Afghanistan and Iraq, was killed, and three other crew members were injured, after an older model CH-53D Sea Stallion made a “hard impact” landing from an altitude of about 300 feet while on a night training flight.

Since then the Marines in Hawaii swapped out the older two-engine CH-53Ds with the newer CH-53E Super Stallion, a more powerful, three-engine helicopter that fulfills a heavy-lift role.

In 2005, 26 Marines and a sailor were from Kaneohe Bay were killed in the crash of a CH-53 Super Stallion in a sandstorm in western Iraq when the crew of the helicopter became disoriented and mistakenly flew the transport chopper into the ground.

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The Associated Press contributed to this report.