(CNN) -- With lead after lead failing to pan out, search and rescue officials said Monday they will expand the search area for the Malaysia Airlines aircraft that vanished three days ago.
The newly expanded search area encompasses a larger portion of the Gulf of Thailand between Malaysia and Vietnam, said Azharuddin Abdul Rahman, director general of the Malaysian Civil Aviation Department.
Nearly three dozen aircraft and 40 ships from 10 countries have so far failed to find any sign of the aircraft.
An oil slick that searchers had thought might be from the plane turned out to be fuel oil typically used in cargo ships, according to Rahman.
Other leads -- reports that a plane door and its tail had been spotted -- turned out to be untrue, he said at an earlier briefing.
"Unfortunately, ladies and gentlemen, we have not found anything that appear to be objects from the aircraft, let alone the aircraft," Rahman said at the earlier briefing.
Authorities are sending ships to investigate a report of debris found south of Hong Kong, but it will likely be Tuesday before authorities know if there is anything to those reports, Rahman said.
No emergency signal has been detected by any search vessel or aircraft. And family members of passengers are being told to prepare for the worst.
So the mysteries surrounding Malaysia Airlines Flight 370 -- and the true identities of some of its passengers -- remain unsolved.
"For the aircraft to go missing just like that ... as far as we are concerned, we are equally puzzled as well," Rahman said.
"We have to find the aircraft."
So far, nothing
Malaysia Airlines Flight 370 took off from Kuala Lumpur shortly before 1 a.m. Saturday (1 p.m. Friday ET). The Boeing 777-200ER, carrying 227 passengers and 12 crew members, went missing while flying to Beijing.
Since then, teams of searchers from Vietnam, China, Singapore, Indonesia, the United States, Thailand, Australia, the Philippines and New Zealand have been working alongside Malaysians to scour the Gulf of Thailand, part of the South China Sea that lies between several Southeast Asian countries.
The focus has now shifted to the Andaman Sea, near Thailand's border, after radar data indicated the plane may have turned around to head back to Kuala Lumpur.
But the pilot apparently gave no signal to authorities that he was turning around.
From 7 a.m. to 7 p.m., planes flew over the vast waters. Ships searched through the night.
The stolen passports
It is perplexing enough that a jetliner seemed to have vanished without a trace. Adding to the mystery is the news that at least two people on board were traveling on passports stolen from an Austrian and an Italian.
The two passengers who used the passports in question appear to have bought their tickets together.
Rahman said Monday that authorities have reviewed security footage from the airport and said the men who traveled on the stolen passports "are not Asian-looking men."
Interpol tweeted Sunday it was examining additional "suspect #passports."
"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," said Interpol Secretary General Ronald K. Noble in a statement.
The passports were reportedly stolen in Thailand, and Thai Prime Minister Yingluck Shinawatra told CNN's "Amanpour" on Monday that police are investigating.
"Initially we don't know about their nationality yet," she said. "But we gave orders for the police to investigate the passport users. Because this is very important to Thailand, to give full cooperation to Interpol in the investigation about the passport users. We are now following this.
The passport mystery raised concerns about the possibility of terrorism, but officials cautioned that it was still too early to arrive at any conclusions.
One possible explanation for the use of the stolen passports is illegal immigration.
There are previous cases of illegal immigrants using fake passports to try to enter Western countries. And Southeast Asia is known to be a booming market for stolen passports.
Five passengers ended up not boarding the aircraft. Their bags were removed and were not on board the jet when it disappeared, Rahman said at Monday's briefing.
Could the plane have been hijacked? "We are looking at every angle, every aspect," Rahman said.
"We are looking at every inch of the sea."
For the relatives of the 227 passengers and 12 crew members, the wait has been agonizing.
Among the passengers, 154 people were from China or Taiwan. The plane was also carrying 38 Malaysians, five Indians and three Americans citizens. Five of the passengers were younger than 5 years old.
In Beijing, family members gathered in a conference room at a hotel complex.
More than 100 people signed a hand-written petition that demanded "truth" from the airline. They also urged the Chinese government to help them deal with Malaysian authorities.
Malaysia Airlines, which was helping family members apply for expedited passports, said it will fly out five relatives of each passenger to Kuala Lumpur.
A fuller picture of what happened may not become available until searchers find the plane and its flight data recorder.
And so far, that hasn't happened.
Posted by Greg Palmer