KUALA LUMPUR, Malaysia (AP) -- Vietnamese searchers on ships worked throughout the night but could not find a rectangle object spotted Sunday afternoon that was thought to be one of the doors of a Malaysia Airlines passenger jet that went missing more than two days ago.
Doan Huu Gia, the chief of Vietnam's search and rescue coordination center, said Monday that four planes and seven ships from Vietnam were searching for the object but nothing had been found.
The Boeing 777 went missing early Saturday morning on a flight from Kuala Lumpur to Beijing with 239 people on board.
The plane lost contact with ground controllers somewhere between Malaysia and Vietnam, and searchers in a low-flying plane spotted an object that appeared to be one of the plane's doors, the state-run Thanh Nien newspaper said, citing the deputy chief of staff of Vietnam's army, Lt. Gen. Vo Van Tuan.
The jetliner apparently fell from the sky at cruising altitude in fine weather, and the pilots were either unable or had no time to send a distress signal, adding to the mystery over the final minutes of the flight.
There are also questions over how two passengers managed to board the ill-fated aircraft using stolen passports.
Interpol confirmed it knew about the stolen passports but said no authorities checked its vast databases on stolen documents before the Boeing jetliner departed Saturday.
Warning "only a handful of countries" routinely make such checks, Interpol secretary general Ronald Noble chided authorities for "waiting for a tragedy to put prudent security measures in place at borders and boarding gates."
On Saturday, the foreign ministries in Italy and Austria said the names of two citizens listed on the flight's manifest matched the names on two passports reported stolen in Thailand.
"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."
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. A person who answered the phone at the agency said she could not comment.
But no authorities in Malaysia or elsewhere checked the passports against the database of 40 million stolen or lost travel documents before the Malaysian Airlines plane took off.
Possible causes of the crash included some sort of explosion, a catastrophic failure of the plane's engines, extreme turbulence, or pilot error or even suicide. Establishing what happened with any certainty will need data from flight recorders and a detailed examination of any debris, something that will take months if not years.
Malaysia's air force chief, Rodzali Daud, said radar indicated that before it disappeared, the plane may have turned back, but there were no further details on which direction it went or how far it veered off course.
"We are trying to make sense of this," Daud said at a news 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 are supposed to inform the airline and traffic control authorities if the plane does a U-turn. "From what we have, there was no such distress signal or distress call per se, so we are equally puzzled," he said.
A total of 34 aircraft and 40 ships from Vietnam, Malaysia, Thailand, Australia, Singapore, Indonesia, China and the United States were deployed to the area where ground controllers lost contact with the plane on the maritime border between Malaysia and Vietnam.
Of the 227 passengers and 12 crew members on board, two-thirds were Chinese, while the rest were from elsewhere in Asia, Europe and North America, including three Americans.
Family members of Philip Wood, a 50-year-old IBM executive who was on board the plane, said they saw him a week ago when he visited them in Texas after relocating to Kuala Lumpur from Beijing, where he had worked for two years.
The other two Americans were identified on the passenger manifest as 4-year-old Nicole Meng and 2-year-old Yan Zhang. It was not known with whom they were traveling.