The military called off its search today for six Marines and a sailor whose helicopter crashed into the ocean and sank a day earlier. It declared the missing dead.

"Our hearts and prayers go out to the Marines and sailors of the 15th Marine Expeditionary Unit, as well as their families and friends," said Lt. Gen. Bruce Knutson Jr. of the Marine Expeditionary Force at Camp Pendleton. "We are doing everything we can to support them in this time of sorrow."

The CH-46 Sea Knight helicopter went down Thursday afternoon about 15 miles off San Diego during a training exercise while flying from one ship to another. Eleven of the 18 Marines aboard were quickly pulled from the chilly water.

The chopper's wreckage is submerged in 3,600 feet of water and has not been located. Only a few pieces of the helicopter have been recovered, said Lt. Scott Bowman, a Marine spokesman. The Coast Guard took to shore two plastic garbage bags containing debris.

The cause of the crash was under investigation.

The Marines were taking part in training maneuvers before their deployment to the Persian Gulf next month.

The helicopter took off from the Bonhomme Richard, an amphibious assault ship, and 14 Marines were ready to rappel 30 feet down a rope onto the Pecos, a Navy tanker.

The San Diego Union-Tribune, citing Navy sources, reported that the helicopter may have approached the Pecos too low and the rope may have snagged an antenna or part of the ship, causing it to crash.

However, NBC News reported that military officials who saw a Navy videotape of the accident said the helicopter's left landing gear got caught in a safety net on deck as it approached the ship. Military officials said it is unclear whether the pilot knew the gear was snagged.

When it tried to lift off, the net pulled down one side of the chopper and it jerked violently and tumbled into the sea, NBC reported.

Lt. David Nevers, a spokesman for Camp Pendleton, where the Marines were based, would not confirm either report. Marine Corps spokeswoman Capt. Aisha Bakkar-Poe in Washington, D.C., said only, "At this point, there's no reason to believe it was a mechanical failure."

The wreckage would be key to any investigation into why the 23,000-pound helicopter went down. Last year, there were two crashes involving Sea Knight helicopters. One killed two sailors in the Mediterranean. Another killed a sailor about 100 miles off Borneo.

Another military accident, this one in Kuwait, claimed the lives of three servicemen. They died when a U.S. Air Force troop transport made an emergency belly landing today in fog and rain. Sixteen others were injured. The aircraft was flying within Kuwait, carrying troops to locations throughout the emirate, when an in-flight emergency was declared, Air Force officials in Washington said.