Vietnamese aircraft spotted what they suspected was one of the doors belonging to the ill-fated Malaysia Airlines Flight MH370 on Sunday, as troubling questions emerged about how two passengers managed to board the Boeing 777 using stolen passports.

The discovery comes as officials consider the possibility that the plane disintegrated mid-flight, a senior source told Reuters.

The state-run Thanh Nien newspaper cited Lt. Gen. Vo Van Tuan, deputy chief of staff of Vietnam's army, as saying searchers in a low-flying plane had spotted an object suspected of being a door from the missing jet. It was found in waters about 56 miles south of Tho Chu island, in the same area where oil slicks were spotted Saturday.

"From this object, hopefully (we) will find the missing plane," Tuan said. Thanh Nien said two ships from the maritime police were heading to the site.

An authority told Reuters that it was too dark to be certain the object was part of the missing plane, and that more aircraft would be dispatched to investigate the site in waters off southern Vietnam in the morning.

Rahman said that the search area has been increased to 50 nautical miles, from 20, and includes 34 aircraft and 40 ships. Aircraft are conducting 12-hour searches, until sundown, while ships are scheduled to continue the search throughout the night.

Meanwhile, Interpol says no country checked its database for information about stolen passports that were used to board the Malaysia Airlines flight that disappeared with 239 people on board Saturday less than an hour after taking off from Kuala Lumpur, Malaysia, bound for Beijing.

In a sharply worded criticism of shortcomings of national passport controls, the Lyon, France-based international police body said information about the thefts of an Austrian passport in 2012 and an Italian passport last year was entered into its database after they were stolen in Thailand.

Interpol said in a statement it was investigating all other passports used to board the flight and was working to determine the "true identities" of the passengers who used the stolen passports.

"I can confirm that we have the visuals of these two people on CCTV," Malaysian Transport Minister Hishammuddin Hussein said at a news conference late Sunday, adding that the footage was being examined. "We have intelligence agencies, both local and international, on board."

Hussein declined to give further details, saying it may jeopardize the investigation. Hussein said only two passengers had used stolen passports, and that earlier reports that the identities of two others were under investigation were not true.

European authorities on Saturday confirmed the names and nationalities of the two stolen passports: One was an Italian-issued document bearing the name Luigi Maraldi, the other Austrian under the name Christian Kozel. Police in Thailand said Maraldi's passport was stolen on the island of Phuket last July.

A telephone operator on a China-based KLM hotline on Sunday confirmed to The Associated Press that "Maraldi" and "Kozel" were both booked to leave Beijing on a KLM flight to Amsterdam on March 8. Maraldi was then to fly to Copenhagen, Denmark, on KLM on March 8, and Kozel to Frankfurt, Germany, on March 8.

She said since the pair booked the tickets through China Southern Airlines, she had no information on where they bought them. The ticket purchases reportedly took place almost simultaneously, and the tickets were numbered consecutively, according to the BBC.

A U.S. official told Fox News that a key priority is clarifying the status of the passports, whether they were lost or stolen, and determining through airport security screening and video who got on the flight under those names.

The statements came as officials said finding the wreckage of the flight is “the utmost priority."

“There is still no sign of the aircraft,” Azharuddin Abdul Rahman, director general of the Department of Civil Aviation, said during a news conference in Kuala Lumpur.

The U.S. Navy sent a warship, the USS Pickney, which was conducting training and maritime security operations off the South China Sea, and a surveillance plane. Singapore said it would send a submarine and a plane. China and Vietnam were sending aircraft to help in the search.

It is not uncommon for it to take several days to find the wreckage of an aircraft floating on the ocean. Locating and then recovering the flight data recorders, vital to any investigation, can take months or even years.

When pressed on reports of fake passports used by at least two passengers on board the flight and the possibility of a terrorist attack, Rahman re-stated that the priority is to find the aircraft and that any probe investigating a terror link is independent of the search mission. Malaysian Prime Minister Najib Razak has also said it is “too early to make any conclusive remarks.”

Earlier, Malaysia’s air force chief told reporters that military radar indicated that the plane may have turned from its flight route before losing contact.

Rodzali Daud didn't say which direction the plane might have taken when it apparently went off route.

"We are trying to make sense of this," he told a media conference. "The military radar indicated that the aircraft may have made a turn back and in some parts, this was corroborated by civilian radar."

Malaysia Airlines Chief Executive Ahmad Jauhari Yahya said pilots were supposed to inform the airline and traffic control authorities if the plane does start to return. "From what we have, there was no such distress signal or distress call per se, so we are equally puzzled," he said.

Vietnamese air force planes spotted two large oil slicks late Saturday in the first sign that the aircraft had crashed. The slicks were each between 6 miles and 9 miles long, the Vietnamese government said in a statement.

But there was no confirmation that the slicks were related to the missing plane, but the statement said they were consistent with the kinds that would be produced by the two fuel tanks of a crashed jetliner.

The plane was carrying 227 passengers, including two infants and 12 crew members when it “lost all contact,” with Subang Air Traffic Control at 2:40 a.m., two hours into the flight, the airline said. The plane was expected to land in Beijing at 6:30 a.m. Saturday.

Around the time the plane vanished, the weather was fine and the plane was already at cruising altitude, making its disappearance all the more mysterious.

Just 9 percent of fatal accidents happen when a plane is at cruising altitude, according to a statistical summary of commercial jet accidents done by Boeing. The plane was last inspected 10 days ago and found to be "in proper condition," Ignatius Ong, CEO of Malaysia Airlines subsidiary Firefly airlines, said at a news conference.

The lack of a radio call "suggests something very sudden and very violent happened," said William Waldock, who teaches accident investigation at Embry-Riddle Aeronautical University in Prescott, Ariz.

The plane "lost all contact and radar signal one minute before it entered Vietnam's air traffic control," Lt. Gen. Vo Van Tuan, deputy chief of staff of the Vietnamese army, said in a statement issued by the government.

U.S. officials said late Saturday that a team of safety experts had been dispatched to Southeast Asia to assist in the investigation. Officials from the National Transportation Safety Board told Fox News that the team, which includes investigators from the agency and technical experts from the Federal Aviation Administration and Boeing, had been sent to the region despite the fact that the plane had not been located due to the lengthy travel time from the U.S. and the team's desire to be in a position to assist local authorities right away. The FBI is also assisting in the search.

Meanwhile, a former intelligence official told Fox News that the information about stolen passports from two adjacent European countries, combined with recent warnings for flights to the United States about the risk of possible shoe bomb attacks, is concerning.

The airline said onboard the plane, there were 152 passengers from China, 38 from Malaysia, seven from Indonesia, six from Australia, five from India and three from the U.S. and others from Indonesia, France, New Zealand, Canada, Ukraine, Russia, Taiwan and the Netherlands.

The U.S. State Department later confirmed in a statement that three Americans were aboard the jetliner.

In the United States, a friend confirmed to the Associated Press that an IBM executive from North Texas named Philip Wood had been aboard the jet. Freescale Semiconductor, a company based in Texas, also confirmed Saturday that 20 of its employees -- 12 from Malaysia and eight from China -- were passengers.

The airline says the plane's pilot is Captain Zaharie Ahmad Shah, a 53-year-old who has been with the airline for over 30 years. The plane's first officer is Fariq Ab.Hamid, a 27-year-old who joined the airline in 2007. Both are Malaysians.

At Beijing's airport, authorities posted a notice asking relatives and friends of passengers to gather at a hotel about nine miles from the airport to wait for further information, and provided a shuttle bus service.

Malaysia Airlines has 15 Boeing 777-200 jets in its fleet of about 100 planes.

The 777 had not had a fatal crash in its 20-year history until the Asiana Airlines crash in San Francisco in July 2013.

Fox News' Catherine Herridge and Dan Gallo, as well as The Associated Press contributed to this report.