The two-day, multi­nation search for a vanished Malaysia Airlines passenger jet has turned up unconfirmed debris but delivered few other clues about one of the most confounding aviation disasters in recent memory.

Searchers via low-flying planes had spotted a rectangular, door-like object on Sunday and something that looked like a tail portion, but by Monday morning, authorities said their ships were unable to relocate both objects, Malaysian officials said at a Monday press conference.

[MONDAY’S LATEST: Hopes raised, and dashed, by debris in ocean; search continues]

Malaysian authorities said Sunday that the plane may have turned around before disappearing from radar without a distress call. But other aviation experts said it probably plummeted suddenly or disintegrated at cruising altitude. A senior U.S. counterterrorism official said there were no indications of terrorism, although nothing had been ruled out. At least two of the 227 passengers boarded with stolen passports.

About 40 ships and 34 aircraft from nine countries have combed Southeast Asian waters for any sign of Malaysia Airlines Flight MH370, and the search expanded Sunday into areas well beyond the plane’s intended flight path.

Friends and relatives of passengers aboard missing Malaysia Airlines plane gather in Beijing to wait for news. (Reuters)

The possible debris was found in an area near two large oil slicks, between six and nine miles long, consistent with fuel left by a downed jetliner. If emergency workers can determine that the flotsam came from the plane, it would mark the first break in an investigation that has left despairing relatives frustrated about the lack of news. On Sunday in Beijing, where the flight was to have landed, some hurled water bottles at a Malaysia Airlines team that arrived to share information.

“Every minute counts, as the plane has lost contact for about 40 hours,” Chinese Foreign Minister Wang Yi told his Malaysian counterpart in a phone conversation described early Monday by Chinese state media.

Malaysia’s state news agency said Monday that samples of the oil have been sent to a chemical lab to determine whether it came from the plane. The results are expected Monday afternoon Malaysia time.

Stolen passports

Investigators focused Sunday on two men who boarded the plane with stolen passports, one of the only leads made public. Malaysian authorities examined closed-circuit television footage of the men at the airport. The international police agency Interpol said in a statement that the passports — Austrian and Italian — had been stolen in Thailand within the past two years and were not checked against an Interpol database as the passengers boarded the red-eye from Kuala Lumpur to Beijing.

“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,” Interpol Secretary General Ronald Noble said in a statement.

Noble expressed frustration that few of Interpol’s 190 member countries “systematically” search the database to determine whether documents being used to board a plane are listed as lost or stolen.

Luigi Maraldi, 37, of Italy and Christian Kozel, 30, of Austria had initially been listed among the plane’s passengers, but both were subsequently found to be safe — and to have had their passports stolen.

(The Washington Post)

Booking information accessed through the KLM Web site showed that the passengers using the passports had adjacent ticket numbers and that both were booked on a subsequent flight from Beijing to Amsterdam. One, traveling under Maraldi’s name, was to continue to Copenhagen and the other to Frankfurt, Germany. Their itineraries were separately confirmed by an employee of China Southern Airlines, which was a code-share partner on the flights and sold them the tickets.

Nevertheless, Clive Williams, a counterterrorism expert at Australia’s Macquarie University, said it seemed unlikely that terrorists would target a Malaysia Airlines flight. Interpol statistics show that 39 million passports were lost or stolen as of the end of last year.

“The stolen passports may or may not be related, but more likely not,” he said by e-mail. “I think it likely that most Asian flights have passengers with stolen passports on board.”

Asked earlier whether terrorism was suspected in the plane’s disappearance, Malaysian Prime Minister Najib Razak said authorities were “looking at all possibilities, but it is too early to make any conclusive remarks.” Airport security procedures were being reviewed, he said, according to the Reuters news agency.

The National Transportation Safety Board in Washington said a team of investigators was en route from the United States to assist with the investigation. U.S. officials said intelligence agencies were examining the possibility of a connection to terrorism.

Transponder went dark

The incident has been hard to piece together in large part because the airliner’s transponder, which broadcasts the plane’s position and location, went dark while the plane was cruising at a steady 35,000 feet. The flight lost contact with Malaysian air traffic control at 1:20 a.m. Saturday, less than an hour after taking off from Kuala Lumpur and as it was completing its ascent. It vanished on the border of the territorial waters of Malaysia and Vietnam, where the Gulf of Thailand meets the South China Sea.

It had been due to land in Beijing at 6:30 a.m. Saturday.

“It can’t vanish from primary radar unless it is a stealth bomber,” said Mikael Robertsson of Flight­radar24, a flight-tracking service. “Everything indicates it must have lost altitude suddenly.”

The fact that the plane was cruising at a steady altitude in decent weather and apparently did not emit a distress signal also were possible indications of a sudden catastrophic event.

“I think this is a criminal act of some kind,” said Scott Hamilton, an aviation expert and founder of Leeham News and Comment in Seattle. “If both engines had failed, the pilots would have had plenty of time to call and say, ‘We have a problem.’ ”

If the plane had broken up on impact with water, search teams probably would have found a fairly concentrated pattern of debris, officials said. But they said an explosion was not the only possible cause: The Boeing 777-200 could have broken up because of mechanical issues. The plane, which was more than 11 years old, had suffered damage to its wing after a minor collision with another aircraft in 2012 but had been fully repaired.

Hamilton said possibilities include a bomb, passengers penetrating the cockpit and seizing the controls and pilot suicide.

But the U.S. deputy national security adviser, Tony Blinken, told CNN that it was too early to say whether foul play was involved.Meanwhile, Chinese officials scrambled Sunday night to get passports and visas to allow passengers’ families in China to fly to Kuala Lumpur, according to ­China’s state-controlled Xinhua News Agency. Malaysia Airlines said it would help the families get to Malaysia as they wait to hear news of the plane’s recovery.

According to Xinhua, Beijing police initiated emergency procedures to enable the relatives to get passports — an often lengthy process in China — within one hour.

Harlan reported from Seoul. William Wan, Liu Liu, Gu Jinglu and Xu Jing in Beijing and Adam Taylor, Robert Barnes, Karen DeYoung, Ian R. Shapira and Sean Sullivan in Washington contributed to this report.