Story highlights China calls for Malaysia to pick up the pace in its search, state media reports

A U.S. intelligence official says stolen passports could be tied to human smuggling

Brother of missing passenger: We're holding out hope, because ... there are no answers"

Underwater search expert: "Crucial time is passing"

What made Malaysia Airlines Flight 370 go off the grid? Where is it now? And could two stolen passports be the key to unraveling what happened?

Questions such as these were more common than answers Monday as searchers kept looking for the Boeing 777 that vanished without a trace three days ago.

As boats and planes scoured the water, chasing a series of leads that proved false, authorities revealed new details in their investigation and speculation surged. Among the latest developments:

-- Two men who apparently boarded the plane with stolen passports -- and the man who bought their tickets -- have become one focus for investigators. So far, authorities haven't said who the men were or why they were on the flight. The FBI was running their thumbprints through a database on Monday after receiving them from Malaysian officials. One U.S. intelligence official noted that the circumstances surrounding the use of the stolen passports follows a pattern similar to human smuggling rings and might not have anything to do with the plane's disappearance.

-- The area that nearly three dozen aircraft and 40 ships from 10 countries are combing has grown. "Now it's a search area hundreds of miles big," Cmdr. William Marks of the U.S. Navy 7th Fleet told CNN.

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Flight 370: Areas being searched

-- It's too soon to know why the plane went missing, but investigators are weighing a number of possibilities and haven't ruled anything out. The possibility of terrorism is still on the table, though the U.S. intelligence official said that's looking less likely.

-- Family members of passengers are being told to prepare for the worst. But the brothers of Philip Wood, an American passenger who was on the plane, told CNN they're relying on faith to keep them going. "We're holding out hope," Tom Wood said, "because as of yet, there are no answers in any of this."

The mysteries surrounding Malaysia Airlines Flight 370 -- and the true identities of some of its passengers -- remain unsolved.

No emergency signal has been detected by any search vessel or aircraft.

"For the aircraft to go missing just like that ... as far as we are concerned, we are equally puzzled as well," said Azharuddin Abdul Rahman, director general of the Malaysian Civil Aviation Department. "We have to find the aircraft."

The stolen passports

It is perplexing enough that a jetliner seems to have vanished without a trace. Adding to the mystery is the news that at least two people on board were traveling on passports stolen from an Austrian and an Italian.

The FBI is running those passengers' thumbprints through its database, a law enforcement official told CNN. The thumbprints were taken at the airport check-in in Kuala Lumpur and were shared with intelligence and law enforcement agencies around the world by the Malaysian government.

Later Monday, a law enforcement official told CNN that information has been shared with various agencies that includes what may be the names of the two men who used stolen passports, and that so far nothing has come up to cause concerns.

Malaysian officials have also shared images of the men with the U.S. government, a U.S. intelligence official said.

"They will compare that to what we have in our terrorist databases. These are lists of people on no-fly lists, people with possible terrorist connections, people we have reasons to be suspicious of," U.S. Rep. Peter King told CNN's "The Lead." "We have these listings, and those names and those biometrics will be compared to those."

According to Thai police officials, an Iranian man by the name of Kazem Ali bought one-way tickets for the two men, describing them as friends who wanted to return home to Europe. While Ali made the initial booking by telephone, either Ali or someone acting on his behalf paid for the tickets in cash, according to police.

But CNN law enforcement analyst Tom Fuentes cautioned against concluding too much from details police apparently learned from a travel agency.

"We don't know if that information is trustworthy. We know that it's a lead and has been provided to the Royal Thai Police. They've furnished it back to the other agencies," said Fuentes, a former FBI assistant director. "But as far as I'm concerned, it still needs to be confirmed who this gentleman was."

Authorities have reviewed security footage from the airport and said the men who traveled on the stolen passports "are not Asian-looking men," Rahman said Monday.

A majority of the 239 people on the plane were from Asia, according to the airline's manifest. There were 154 people from China or Taiwan and 38 people from Malaysia aboard.

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The Italian whose name was on the plane's manifest, Luigi Maraldi, told reporters in Thailand over the weekend that he'd reported his passport stolen in August.

Interpol tweeted Sunday it was examining additional "suspect #passports."

"Whilst it is too soon to speculate about any connection between these stolen passports and the missing plane, it is clearly of great concern that any passenger was able to board an international flight using a stolen passport listed in INTERPOL's databases," said Interpol Secretary General Ronald K. Noble in a statement.

The passports were reportedly stolen in Thailand, and Thai Prime Minister Yingluck Shinawatra told CNN's "Amanpour" on Monday that police are investigating.

"Initially we don't know about their nationality yet," she said. "But we gave orders for the police to investigate the passport users. Because this is very important to Thailand, to give full cooperation to Interpol in the investigation about the passport users. We are now following this."

So far, nothing

Malaysia Airlines Flight 370 took off from Kuala Lumpur shortly before 1 a.m. Saturday (12 p.m. Friday ET).

The plane disappeared somewhere between Malaysia and Vietnam. Since then, teams of searchers from Vietnam, China, Singapore, Indonesia, the United States, Thailand, Australia, the Philippines and New Zealand have been working alongside Malaysians to scour the Gulf of Thailand, part of the South China Sea that lies between several Southeast Asian countries.

The focus has now shifted to the Andaman Sea, near Thailand's border, after radar data indicated the plane may have turned around to head back to Kuala Lumpur.

But the pilot apparently gave no signal to authorities that he was turning around.

Investigators have said several leads have turned out to be dead ends.

An oil slick that searchers had thought might be from the plane turned out to be fuel oil typically used in cargo ships, according to Rahman.

Other leads -- reports that a plane door and its tail had been spotted -- also turned out to be untrue.

China called on Malaysia to pick up the pace in its search, the state-run Xinhua news agency reported.

"We hope Malaysia can fully understand China, especially the mood of the Chinese passengers' families and speed up investigation, search and rescue efforts," Foreign Ministry spokesman Qin Gang told reporters, according to Xinhua.

Search and rescue officials said Monday they were expanding the search area to encompasses a larger portion of the Gulf of Thailand between Malaysia and Vietnam.

Authorities are sending ships to investigate a report of debris found south of Hong Kong, but it will likely be Tuesday before authorities know if there is anything to those reports, Rahman said.

From 7 a.m. to 7 p.m., planes flew over the vast waters. Ships searched through the night.

"Every day that goes by, it makes the search area much, much larger," said David Gallo, who helped lead the search for the wreckage of Air France Flight 447 after that plane crashed in 2009.

Looking for wreckage and flight data recorders in the water is no easy task, he told CNN's "The Situation Room."

"We've only explored about 7% of the world beneath the sea, and there's a reason for that. It's slow going, and it's difficult. So, with every day that passes by, crucial time is passing."

Terrorism concern

The passport mystery raised concerns about the possibility of terrorism, but officials cautioned that it was still too early to arrive at any conclusions.

One possible explanation for the use of the stolen passports is illegal immigration.

There are previous cases of illegal immigrants using fake passports to try to enter Western countries. And Southeast Asia is known to be a booming market for stolen passports.

Five passengers ended up not boarding the aircraft. Their bags were removed and were not onboard the jet when it disappeared, Rahman said at Monday's briefing.

Could the plane have been hijacked? "We are looking at every angle, every aspect," Rahman said.

"We are looking at every inch of the sea."

There has been some speculation that the flight might have been a test run for a terrorist organization planning a later attack.

The incident has some similarities to such incidents in the past, such as the 1994 bombing of a Philippine jetliner that investigators later learned was a test run for a wider plot to bomb numerous airliners, former U.S. Department of Transportation Inspector General Mary Schiavo told CNN on Monday.

But John Magaw, a former Transportation Security Administration official and U.S. Secret Service director, said his best guess is the Malaysia Airlines flight was not a test.

"They've already done the dry run," he said. "This was the actual flight."

Analysts warned that it's far too soon to know why the plane went missing or what caused it.

"We have speculation run amok, because we have no facts," said Michael Goldfarb, a former chief of staff for the Federal Aviation Administration. "Speculation on the cause is always wrong, because it's a unique accident. It rarely happens.

"And I do believe that they will find the so-called black boxes. ... They'll find where the plane is."

Agonizing wait

For the relatives of the 227 passengers and 12 crew members, the wait has been agonizing.

"As of yet, we know as much as everyone else," Tom Wood told CNN's "AC360" Monday. "It seems to be getting more bizarre, the twists in the story, where they can't find anything. So we're just relying on faith."

In Beijing, family members gathered in a conference room at a hotel complex.

More than 100 people signed a hand-written petition that demanded "truth" from the airline. They also urged the Chinese government to help them deal with Malaysian authorities.

Malaysia Airlines, which was helping family members apply for expedited passports, said it will fly out five relatives of each passenger to Kuala Lumpur.

A fuller picture of what happened may not become available until searchers find the plane and its flight data recorder.

And so far, that hasn't happened.