The mystery, confusion and disarray surrounding the missing Malaysia Airlines plane worsened on Tuesday, after a military official suggested the aircraft had not only been turned around but had flown back across the Malay peninsula.

Flight MH370 was bound for Beijing when it vanished in the early hours of Saturday with 239 on board. Until Tuesday, the last known contact with the flight was thought to be at about 1.20am – 40 minutes after take-off from Kuala Lumpur – after the plane crossed Malaysia's east coast and was flying over the South China Sea towards Vietnam.

But the air force chief, Tan Sri Rodzali Daud, said the plane was detected at 2.40am near Pulau Perak, an island in the Malacca Strait, several hundred kilometres north of Kuala Lumpur.

"After that, the signal from the plane was lost," he told the Berita Harian, a Malay-language newspaper.

A military official told Reuters: "[The plane] changed course after Kota Bharu [on the east coast] and took a lower altitude. It made it in to the Malacca Strait."

Pilots are supposed to inform their airlines and air traffic control if they change course. MH370 never did so; nor did crew issue a distress call.

It is unclear why the contact from the west coast, if correct, was not made public until now. Asked on Monday why crews were searching the strait, the country's civil aviation chief, Azharuddin Abdul Rahman, told reporters: "There are some things that I can tell you and some things that I can't."

Malaysian officials have given ambiguous, inaccurate and at times directly contradictory information since the aircraft's disappearance, raising concerns among families of the passengers.

Adding to the confusion, Tengku Sariffuddin Tengku Ahmad, a spokesman for the prime minister's office, said in a telephone interview that he had checked with senior military officials, who told him there was no evidence that the plane had recrossed the Malaysian peninsula, only that it may have attempted to turn back.

"As far as they know, except for the air turnback, there is no new development," he said, adding that the reported remarks by the air force chief were "not true".

The head of the international police agency said terrorism seemed a less likely possibility as the expanding hunt for the aircraft entered its fourth day.

"The more information we get, the more we are inclined to conclude it is not a terrorist incident," said Ronald Noble of Interpol. He said the two Iranian passengers travelling on stolen passports were unlikely to have been terrorists.

Malaysian authorities said their minds were open to all possibilities. The inspector-general of police, Tan Sri Khalid Abu Bakar, said officers were examining whether hijacking, sabotage or the crew and passengers' personal or psychological problems could be responsible.

"Other than mechanical problems, these are the main areas of concern," he said. Asked what he might mean by personal problems, he gave as the example of someone who had bought an insurance policy to benefit relatives. He did not specify what he meant by psychological issues, but some aviation specialists have cited the example of the Egypt Air crash in 1990. The disaster was widely ascribed to pilot suicide, although Egypt never accepted that finding.

Asked about the plane, at an event in Washington, John Brennan, director of the Central Intelligence Agency, said terrorism could not be ruled out.

Dan Macchiarella, chair of the aeronautical science department at Embry-Riddle Aeronautical University, Daytona Beach, Florida, noted that if a jet at such a high altitude had engine problems it might still be able to glide "for a very, very long distance", given the altitude and speed.

But he added: "It's pretty baffling. Whatever happened on that flight deck, the pilots did not do what pilots do. They aviate, they navigate and they communicate. If something happens at altitude, the first thing they want to do is … squawk emergency."

His colleague, Les Westbrooks, an associate professor of aeronautical science at Embry-Riddle, said it was possible, though highly unlikely, that the radio systems had failed. He said he suspected catastrophic failure and doubted that the aircraft turned around because if it had time to do so it would have had time to make and could have made a radio call.

He added that it was possible something happened but the pilots maintained control initially, "continuing on to try to land in possibly Vietnam or somewhere else … [and] then the situation got worse".

If the plane did turn and reach the Malacca Strait then the transponder, which should communicate with civil radar, did not appear to have been working. Experts say it is highly unusual for the device simply to fail, but that it might do so if the antenna outside the plane was damaged.

Another possibility is the crew turned it off, for unknown reasons. "If the crew knew that they were flying in a non-radar environment, they might very well turn the transponders off. Not necessarily that that's standard operating procedure – but they're up there, [they might just think] nobody's interrogating this, let's just turn it off," he said, suggesting the crew could have sought "to save wear and tear on the electronics".

The other possibility is that the plane blew up at the last point where its transponder communicated with the radar, he said.

The west coast reading was on military radar, which does not rely on communicating with a transponder as civilian radar does. That may explain the uncertainty over whether MH370 was detected or not.

"Military radar sends out a signal and paints a skin. It gets the type, speed and altitude of aircraft," noted Macchiarella.

In addition to the multinational effort to search more areas of sea off the east and west coasts of the Malay peninsula, the hunt for clues has spread to land.

Malaysia Airlines said authorities were searching the Malaysian peninsula, while the Vietnamese military said its units were hunting for any sign that the aircraft might have crashed into remote mountains or uninhabited jungle areas in its territory.

Around two-thirds of the 227 passengers on board were Chinese. Family members waiting at a hotel in Beijing were still clinging to hope, despite being warned to prepare for the worst.

"I hope it is a hijacking, then there will be some hope that my young cousin has survived," one man told AFP news agency. "My uncle and aunt had an emotional breakdown: they are not eating, drinking and sleeping and could not face coming here."

Malaysia Airlines said in a statement: "We regret and empathise with the families and we will do whatever we can to ensure that all basic needs, comfort and psychological support are delivered. We are as anxious as the families to know the status of their loved ones."