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Crews are continuing the search for a missing U.S. military chopper and its eight passengers after it vanished Tuesday somewhere in the cloaked and craggy Himalayan hillside of Nepal.

But there remains no contact from the UH-1Y Huey, military officials say, even though the helicopter was equipped with a GPS device, radio and emergency beacon. Its unexplained disappearance has left even experienced helicopter pilots perplexed.

"It's baffling," retired Army helicopter pilot Jim Weatherill told NBC News on Thursday. "Why isn't the emergency beacon transmitting a frequency to where they might be?"

The Huey vanished late Tuesday night following a magnitude-7.3 aftershock that rocked Nepal earlier that day, killing at least 96 people. The chopper was on its way to deliver aid in the hard-hit district of Dolakha, east of Kathmandu. Six Americans and two Nepalese service members were on board.

Defense Department officials have said there were no reports of smoke or a loud bang indicating that the aircraft crashed. Two Marine MV-22B Osprey tiltrotor aircraft have been combing a search area.

Army Col. Steve Warren, a Pentagon spokesman, said a helicopter from India reported hearing about a possible fuel problem with the Huey. The Marines had just dropped off their supplies in one location and were on their way to a second stop when they lost contact.

Even if there was fuel or engine trouble and the helicopter had to make an emergency landing, it's puzzling that no one on board would have radioed, experts say.

"Whatever happened to them, it had to have happened quickly," said Weatherill, a pilot during the Vietnam War who has experience with Huey models.

If there was no flat surface to touch down on, the crew could have made an emergency landing on water, Weatherill said, in which case hearing the emergency beacon would be much more difficult depending on how submerged the aircraft became.

"The helicopter doesn't float," Weatherill added, "so the guys would have had to prepare for the emergency landing and have the doors open so they could swim out."

Theoretically, the beacon's signal would still go off like a siren and be picked up by someone monitoring the frequency. But detecting the signal also requires a line of sight that could be disrupted because of the mountainous terrain of the Himalayas, military officials caution.

Weatherill said if the aircraft is on land, it's only a matter of time before it's found. Helicopter missions include tailored flight plans, and the search teams would be focusing on a specific zone, while factoring in how much fuel the aircraft would have had before needing to land.

For now, it's all speculation. Military officials say there's a possibility the service members are alive and in an area where they simply can't communicate. Still, "it's going to be very difficult to figure out what happened without finding any presumed wreckage first," Weatherill said.

IN-DEPTH

— Erik Ortiz