Kuala Lumpur, Malaysia (CNN) -- The mysteries surrounding the disappearance of Malaysia Airlines Flight 370, and the true identities of some of its passengers, are as deep as the southeast Asian waters where multinational search teams are searching for the jet.
Navies from two of Malaysia's neighbors were pursuing new leads as Sunday turned into Monday in southeast Asia.
Vietnam's navy has spotted a floating object about 80 kilometers (50 miles) southwest of Vietnam's Tho Chu Island, which is located off the country's southwest coast in the Gulf of Thailand, Vietnam National Search and Rescue Committee Spokesman Hung Nguyen told CNN. The object was spotted by a Vietnamese navy rescue aircraft at about 7:30 a.m. ET Sunday (6:30 p.m. local time). Due to the dark, the navy aircraft could not get close enough to identify the floating object, and was recalled to base. Three search and rescue boats have since been deployed to that location.
Meanwhile, Thailand's navy is shifting its focus in the search away from the Gulf of Thailand and the South China Sea, Thai Navy Rear Adm. Karn Dee-ubon told CNN on Sunday. The shift came at the request of the Malaysians, who are looking into possibilities the plane turned around and could have gone down in the Andaman Sea, near Thailand's border, Karn said.
The Andaman Sea lies to the west of a narrow strip of Thailand that ends in the Malaysian peninsula, while the Gulf of Thailand lies to the east of that Thai isthmus.
One promising lead has turned out to be a dead end. A "strange object" spotted by a Singaporean search plane late Sunday afternoon is not debris from the missing jetliner, a U.S. official familiar with the issue told CNN on Sunday.
A U.S. reconnaissance plane "thought it saw something like debris but it was a false alarm," said the official, who spoke on condition of anonymity.
By the end of the day Sunday, more than 40 planes and more than two dozen ships from several countries were involved in the search. Two reconnaissance aircraft from Australia, and one plane and five sea vessels from Indonesia were the latest additions, Azharuddin Abdul Rahman, the director general of civil aviation in Malaysia, told reporters Sunday. In addition, the Chinese navy dispatched a frigate and an amphibious landing ship, according to a online post by China's navy. Those ships are expected to arrive on site Monday morning (Sunday night ET).
Those reinforcements join the rescue teams already scouring the South China Sea, near the Gulf of Thailand, for any sign of where the flight, operated by Malaysia's flagship airline, might have gone down, Malaysian authorities said.
The area in focus for most of the search, about 90 miles south of Tho Chu Island, is where a Vietnamese plane reportedly spotted oil slicks that stretched between 6 and 9 miles.
Malaysian authorities have not yet confirmed the report of the oil slicks, which came from Vietnam's official news agency.
Big questions far outweigh the few fragments of information that have emerged about the plane's disappearance.
What happened to the plane? Why was no distress signal issued? Who exactly was aboard?
The flight may have changed course and turned back toward Kuala Lumpur, Malaysian military officials said at a news conference Sunday.
But the pilot appears to have given no signal to authorities that he was turning around, the officials said, attributing the change of course to indications from radar data.
As the search continues, the agonizing wait goes on for relatives of the 227 passengers and 12 crew members on board the plane. Video from Reuters showed Malaysia Airlines personnel in Beijing, where Flight 370 was headed, helping family members apply for expedited passports so they could fly to Kuala Lumpur early this week.
Among the passengers, there were 154 people from China or Taiwan; 38 Malaysians, and three U.S. citizens. Five of the passengers were younger than 5 years old.
Click here for more on the story. Contributions to CNN reporters Jim Clancy and Mark Morgenstein.