Five hundred miles north of Alaska, a group of shipmates from the Coast Guard cutter Healy tossed a football on the blue-and-white, diamond-hard Arctic ice.

Others snapped panoramic photos and took walks during the two-hour break, stretching their legs after a month aboard the 420-foot icebreaker.

Lt. Jessica Hill and Boatswain's Mate Steven Duque seized the chance for a training dive and slipped into a patch of open water near the Healy's bow. A team held ropes attached to the divers, lest they become disoriented under the ice. Several research scientists watched from the deck.

But no one knows what happened on the other end of those ropes on that cold, brilliant summer day — except that both divers died.

The Coast Guard has started two investigations, relieved the Healy's captain, pulled all diving equipment off the ship and suspended all polar diving. But nothing has been said about what might have killed Hill, 31, and Duque, 22, on Aug. 17, or when the investigations will conclude.

"We can get no word whatsoever, and that's tough," Hill's father, William Hill Jr., said. "We can't even get the death certificates."

The Healy was on a research mission backed by the National Science Foundation. On board were three dozen scientists collecting data that would help them map the ocean floor and study the Earth's crust to better understand earthquakes, tsunamis and plate tectonics.

Hill, the ship's marine science officer and a native of St. Augustine, Fla., was an experienced civilian diver before she joined the Coast Guard about four years ago. Her shipmates described her as a fun-loving officer who, during a trip to the North Pole last year, posed on the ice in a bikini by a red and white striped pole.

Duque, whose responsibilities included keeping the Healy's decks in order, operating machinery and driving launch boats, was from Miami. Colleagues said he was exceedingly professional and inspired others to take their jobs seriously.

Both attended the Navy's dive school, which is required of all Coast Guard divers.

The pair had been underwater for about 10 minutes, estimated Harm Van Avendonk, a University of Texas geophysics researcher, and something appeared to be wrong.

"I saw people from the bow looking intently down on the ice, and I sensed immediately that they didn't look relaxed," he said. "It was taking a long time for the divers to reappear."

In a blur, the crew's training took over, several witnesses said.

The divers were pulled up by the ropes. Blankets and stretchers were rushed onto the ice, and EMTs immediately began performing CPR. The divers were carried to the ship's sick bay, where they were pronounced dead roughly two hours after the dive.

"What I can tell you is this: These people were very well trained. Every time we did something we had to have a safety briefing," said Steve Stevenoski, a high school teacher from Wisconsin Rapids, Wis., who was videotaping the frozen seascape when he heard shouts from the dive support team.

"There was an accident that was completely unforeseen," he said.

According to Coast Guard protocol, they would have created a "dive profile," detailing who was diving, how far down they were going and how long they would spend at various depths.

Typically such plans are drawn up by a ship's dive officer, though the captain is ultimately responsible for the safety of divers. That could explain why Capt. Douglas Russell was relieved of command less than two weeks later. Vice Adm. Charles D. Wurster, commander of the Coast Guard in the Pacific, said he had lost confidence in Russell.

The only signs of the tragedy during a recent tour of the ship were a grief counseling pamphlet on a table in the scientists' lounge and the locked and empty room where dive equipment was stored. The equipment was shipped to the Navy's dive school in Panama City, Fla., for examination.

One Coast Guard investigation is focusing on the root cause in hopes of preventing future accidents; the other is a broader administrative investigation that could result in findings of responsibility.

One investigator, a lieutenant, said Hill and Duque were the first Coast Guard divers to die under water since the 1970s.

The Coast Guard described the dive as routine, but any dive in frigid waters beneath 4-feet-thick ice poses serious dangers. The cold can numb the extremities. Divers typically wear dry suits, which use air to help determine buoyancy. Such suits can balloon during ascents as pressure decreases — if the diver doesn't release the air quickly enough, he or she can shoot toward the surface and crash into the ice.

They also must use equipment that can handle the cold, such as breathing regulators outfitted with rubberized covers filled with antifreeze.

The deaths were hard on the Healy's crew of 75, said Ensign Stephen Elliott, who was on the ice as part of the dive support team that day.

"These are people you watch movies with, eat with, joke around with," he said. "It's hard to explain to someone who doesn't live on a ship what it's like to be a shipmate. They were incredible shipmates."