The investigation into the fate of Malaysia Airlines Flight MH370 widened Monday, as authorities questioned travel agents about two men who boarded the plane with stolen passports and looked into why five individuals who checked in for the flight did not board.

Meanwhile, officials continued to scour the Southeast Asian waters for the Boeing 777 that disappeared nearly three days ago after an earlier report that an oil slick may have come from the plane proved untrue.

Malaysia civil aviation chief Azharuddin Abdul Rahman said the baggage of five passengers who had checked in for the flight but did not board the plane was removed before it departed. Airport security was strict according to international standards, surveillance has been done and the airport has been audited, he added.

A U.S. official told Fox News that the flight manifest has been run against the TIDE database -- a broad database that includes known and suspected terrorists -- and there were no obvious red flags.

Investigators have images of the two passengers who used stolen passports to board the plane, and they think they have a positive ID on one of the passengers, the official said. There is now outreach to foreign intelligence partners for additional data, including photos to verify the name.

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According to the official, a single claim of responsibility has been identified and is being checked out, but it was not from a known terrorist group and not a lot of stock was being put into the claim.

Rahman said authorities were looking at the possibility the two men were connected to a stolen passport syndicate.

A senior Malaysian police official told Reuters Monday that there have been past incidents of people being caught trying to fly out of Kuala Lumpur with explosives and fake passports.

"We have stopped men with false or stolen passports and carrying explosives, who have tried to get past KLIA [airport] security and get on to a plane," the police official said. "There have been two or three incidents, but I will not divulge the details."

Interpol said it knew about the stolen passports, but said no authorities checked its vast databases on stolen documents before the Boeing jetliner departed Saturday.

The thefts of the two passports -- one belonging to Austrian Christian Kozel and the other to Luigi Maraldi of Italy -- were entered into Interpol's database after they were stolen in Thailand in 2012 and last year, the police body said.

Electronic booking records show that one-way tickets with those names were issued Thursday from a travel agency in the beach resort of Pattaya in eastern Thailand. Thai police Col. Supachai Phuykaeokam said those reservations were placed with the agency by a second travel agency in Pattaya, Grand Horizon.

Thai police and Interpol officers questioned the owners. Officials at Grand Horizon refused to talk to The Associated Press.

Police Lt. Col. Ratchthapong Tia-sood said the travel agency was contacted by an Iranian man known only as "Mr. Ali" to book the tickets for the two men.

"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," he said.

Interpol said it and national investigators were working to determine the true identities of those who used the stolen passports to board the flight. White House Deputy National Security Adviser Tony Blinken said the U.S. was looking into the stolen passports, but that investigators had reached no conclusions.

Possible causes of the apparent crash include an explosion, catastrophic engine failure, terrorist attack, extreme turbulence, pilot error or even suicide, according to experts, many of whom cautioned against speculation because so little is known.

By early Tuesday morning, the search had broadened to involve at least 34 aircraft and 40 ships from several countries scouring a 115-mile radius from the point the plane vanished from radar screens between Malaysia and Vietnam early Saturday with no distress signal.

Searchers were dealt a double blow Monday when a floating yellow object that was believed to be a life raft from the plane when it was spotted Sunday turned out to be the moss-covered cap of a cable reel, according to Vietnam's civil aviation authority. Earlier in the day, Vietnamese officials said that they had not been able to locate a rectangular object that appeared to be one of the plane's doors.

Although recovering black boxes are a priority, U.S. officials told Fox News that a piece of fuselage can be just as telling. It can be tested for explosive residue and the damage, such as blowout marks, can show where the explosion came from.

A team of American experts was en route to Asia to be ready to assist in the investigation into the crash. The team includes accident investigators from the National Transportation Safety Board, as well as technical experts from the Federal Aviation Administration and Boeing, the safety board said in a statement.

Finding traces of an aircraft that disappears over sea can take days or longer, even with a sustained search effort. Depending on the circumstances of the crash, wreckage can be scattered over a large area. If the plane enters the water before breaking up, there can be relatively little debris.

Meanwhile, hundreds of distraught relatives were gathered in a hotel in Beijing, waiting to be flown to Malaysia. Of the 227 passengers, two-thirds were Chinese. There were also 38 passengers and 12 crew members from Malaysia, and others from elsewhere in Asia, Europe and North America, including three Americans.

"We accept God's will,” said Selamat Omar, a Malaysian whose 29-year-old son Mohamad Khairul Amri Selamat was heading to Beijing for a business trip. Whether he is found alive or dead, we surrender to Allah.”

Fox News’ Catherine Herridge and The Associated Press contributed to this report.