There was a possibility that the missing Beijing-bound Malaysia Airlines flight MH370 had to make an "air turn back", Malaysia's Acting Transport Minister Hishammuddin Hussein said on Sunday.
He said they were investigating all possible reasons for this around turn including the possibility of terrorism. Malaysia was working with foreign intelligence agencies including the FBI and counter-terrorism units on this.
He said at a press conference on Sunday that they were also investigating reports that at least two passengers had boarded the flight with false passports.
Two men - an Italian and an Austrian - have come forward to say that they were not on the flight although their names appears on the manifest. Their passports had been reported lost in Thailand. The manifest only lists names of passengers who have boarded the flight.
Latest reports state that another two passengers with European passports did not board the flight despite their names appearing on the list.
Mr Hishammuddin refused to give their names or nationalities but said all four were being investigated.
"Not just the four, the whole manifest is being investigated now. It's not concluded that people with stolen passports were on board, it's still being investigated," he said.
He declined to say if there was a security lapse.
"Our main focus is to find the aircraft. There is a possibility that the plane had to make an air turn back so we have to expand the search area," he said.
The search area had expanded to the west coast of Malaysia.
He said Malaysia's Department of Civil Aviation and the Royal Malaysian Air Force were looking into the reasons for the turn back, and would announce their findings later.
He also said he spoke to Singapore's Defence Minister Dr Ng Eng Hen on Saturday who agreed to deploy three Singapore vessels to help with the search.
They are MV Swift Rescue, a submarine support and rescue vessel, RSS Vigour, a missile corvette with search capabilities, and RSS Steadfast, a frigate with sonar search capabilities.
"We are very grateful for the help and response from our neighbouring states," he said.
Mr Hishammuddin confirmed that oil slicks were found in the area where the plane last had contact with Malaysia's air traffic control but no debris was found.
It also was not confirmed that debris was found off the east coast of Malaysia.
The Malaysia Airlines flight MH370 lost contact with Subang Air Traffic Control 50 minutes after departing KLIA at 12.41am on Saturday.
It was due to land in Beijing at 6.30am, and had enough fuel to fly up to 8.30am. The plane did not issue a distress signal nor was there bad weather when it disappeared.
The plane was last on the radar at approximately 120 nautical miles off Kota Baru, in the South China Sea, in the airspace bordering Vietnam. Malaysian air traffic control had just handed over control to the Vietnamese authorities when they lost contact.
The flight was carrying a total number of 239 passengers and crew - comprising 227 passengers (including 2 infants) and 12 crew members.
There were 15 nationalities on board, the majority Chinese nationals and Malaysians. No Singaporeans were on board.
The Vietnamese authorities are searching in their waters while Malaysia scoured their own. Other countries involved in the search are China, the Philippines, Singapore and the US.
Malaysia said the search would go on round the clock until a decision was made to call it off.
Flight MH370 was operated on a Boeing 777-200 aircraft, a codeshare with China Southern Airlines. The plane was 11 years 10 months old.
The next press conference is scheduled at 1pm.