Suspected fragments from the missing Malaysia Airlines plane carrying 239 people have apparently been found off Vietnam.
But officials have said it was too dark to be certain the objects were from Flight MH370, which had 239 people on board.
Vietnamese authorities searching waters for the missing Boeing 777 jetliner spotted an object Sunday that they suspected was one of the plane's doors, as international intelligence agencies joined the investigation into two passengers who boarded the aircraft with stolen passports.
Security services are said to be investigating whether the plane was destroyed in a terror attack.
More than a day and half after the plane went missing, no confirmed debris from the plane had been found, and the final minutes before it disappeared remained a mystery.
The plane lost contact with ground controllers somewhere between Malaysia and Vietnam after leaving Kuala Lumpur early Saturday morning for Beijing.
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 90 kilometers (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.
The missing plane 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 — unusual circumstances under which a modern jetliner operated by a professional airline would crash.
Malaysian military officials said on Sunday that the plane may have turned back from its scheduled route shortly before vanishing from radar screens, further deepening the mystery surrounding its fate.
"We are trying to make sense of this," Malaysia's air force chief, Rodzali 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.
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Authorities were checking on the identities of the two passengers who boarded the plane with stolen passports. 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.
In addition to the plane's sudden disappearance, which experts say is consistent with a possible onboard explosion, the stolen passports have strengthened concerns about terrorism as a possible cause. Al-Qaida militants have used similar tactics to try and disguise their identities.
Other possible causes would seem just as likely at this stage, including 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.
Meanwhile, the multinational search for the missing plane was continuing. A total of 34 aircraft and 40 ships have been deployed to the area by Malaysia, Thailand, Australia, Singapore, Indonesia, China and the United States, in addition to Vietnam's fleet.
After more than 30 hours without contact with the aircraft, Malaysia Airlines told family members they should "prepare themselves for the worst," Hugh Dunleavy, the commercial director for the airline, told reporters.