BEIJING — An international hunt for clues in the disappearance of a Malaysia Airlines passenger jet intensified Monday, with dozens of ships searching a vast expanse of sea and investigators chasing down leads. But authorities acknowledged that they were stymied.

“This unprecedented missing aircraft mystery — as you can put it — it is mystifying,” Azharuddin Abdul Rahman, director general of Malaysia’s Department of Civil Aviation, said at a news conference in Kuala Lumpur.

The U.S. Navy dispatched a second ship Monday to assist an emergency operation in the Gulf of Thailand and South China Sea that has grown to involve at least 40 other vessels and 34 aircraft from 10 countries. But as in the previous two days of searching, no wreckage from Malaysia Airlines Flight MH370 appeared.

In Thailand, officials interviewed travel agents in the beach resort of Pattaya, where tickets were apparently issued for two men who later boarded the flight with stolen passports, according to the Associated Press. The two men’s fake identities had raised the possibility that a terrorist attack brought down the Boeing 777, which was carrying 227 passengers from Kuala Lumpur to Beijing when it vanished Saturday.

But U.S. and other officials say they have found no evidence of terrorist involvement.

Senior American officials dismissed reports that a group called the Chinese Martyrs’ Brigade had asserted responsibility for the plane’s disappearance. “No group by that name has been previously identified, and it is not clear who is behind the claim,” said a U.S. intelligence official who was not authorized to be quoted by name.

Numerous scenarios

In a vacuum of evidence about what went wrong aboard the flight, speculation turned to the possibility of pilot suicide, an extraordinarily rare occurrence.

“You have to ask the question,” said a U.S. aviation official who was not authorized to discuss the matter publicly.

The Malaysia Airlines flight reportedly was being tracked by radar when its transponder went dark. There were no radio transmissions to indicate that anything was amiss aboard the plane. The transponder signals and radio communication are controlled by the pilot.

There have been two cases in recent years in which a pilot or crew member is believed to have intentionally caused a plane to crash: the disaster involving SilkAir Flight 185, which spiraled into the ground in Indonesia in 1997, killing 97 passengers and seven crew members; and the crash of EgyptAir Flight 990, which plunged into the Atlantic south of Nantucket in 1999, killing 217 people.

But Steve Marks, a Miami aviation lawyer who represented families in two instances in which an airliner plummeted from cruising altitude, pointed to a mechanical failure as the most likely cause of the Malaysia tragedy.

“There can be a mechanical problem that can occur at altitude, where the pilots are unable to report the failure and the aircraft is lost on radar,” he said.

Nonetheless, he said, the failure of all communications from the Malaysia Air flight made it “the most mysterious” crash in his recollection.

A ‘creeping line’

On Monday, the USS Kidd joined the USS Pinckney surveying the area where the plane is presumed to have crashed, officials said. The ships and their Seahawk helicopters were searching in a zigzag pattern known as a “creeping line,” Navy officials said.

“Just from the air, we can see things as small as almost the size of your hand or a basketball. It’s not a matter of if we can see it. It’s an extremely large area,” a spokesman for the U.S. Navy’s 7th Fleet, Cmdr. William Marks, said in an interview with the BBC.

On Monday, hopes briefly centered on a rectangular orange object that authorities said might have been a life jacket. But when a Vietnamese helicopter recovered the piece of flotsam, it was identified as “a moss-covered cap of a cable reel,” the Civil Aviation Authority of Vietnam said on its Web site.

It was not the first time hopes have been dashed in the search.

Late Sunday, Vietnamese authorities said one of their aircraft had spotted a rectangular object that could have been an inner door from the plane. By Monday, ships and planes could not locate the object. Meanwhile, sightings of what had resembled a piece of the plane’s tail turned out to be logs tied together, Malaysian authorities said.

Two oil slicks, between six and nine miles long, consistent with fuel left by a downed jetliner, were tested and found to not be connected to the plane.

In Thailand, police Lt. Col. Ratchthapong Tia-sood said an Iranian man known only as “Mr. Ali” had contacted the Grand Horizon travel agency in Pattaya to book flight tickets for the two men using stolen passports, according to the AP. Grand Horizon asked another agency in the resort town to issue the one-way tickets, the AP reported.

“We have to look further into this Mr. Ali’s identity, because it’s almost a tradition to use an alias when doing business around here,” the police officer told the AP.

On Monday, Azharuddin said closed-circuit television footage showed that the two men passed through normal security checks at the airport. He suggested they were dark-skinned and not of Asian appearance. Officials also said they have shared “biometric and visual” information about the men with U.S. intelligence agents.

The men were using passports stolen in Thailand in 2012 that belonged to Luigi Maraldi, 37, of Italy and Christian Kozel, 30, of Austria.

Azharuddin said five other passengers checked in for the flight but never boarded. He said their baggage was removed before the plane took off.

Ashley Halsey, Ernesto Londoño and Adam Taylor in Washington, Chico Harlan in Seoul and Liu Liu, Gu Jinglu and Xu Jing in Beijing contributed to this report.