Confirmation that a missing Malaysian airliner was deliberately diverted suggests several scenarios that have sharpened scrutiny of the passengers and cockpit crew, with police reportedly searching the pilot's home.
Prime Minister Najib Razak announced Saturday that satellite and radar data clearly indicated the plane's automated communications had been disabled and the plane then turned away from its intended path and flown on for hours.
"These movements are consistent with deliberate action by someone on the plane," he said, adding that investigators had consequently "refocused their investigation into crew and passengers on board."
Flight MH370 was under the command of Captain Zaharie Ahmad Shah, and his First Officer Fariq Abdul Hamid.
Malaysian reporters told AFP they witnessed police enter Zaharie's home on Saturday, spending two hours there. Police declined comment to AFP.
The 53-year-old had assembled his own flight simulator at home, according to online tributes describing his passion for flying.
Since the September 11, 2001 terror attacks, the International Civil Airline Organisation has mandated high security standards for plane cockpits.
Cockpit doors -- reinforced to withstand bullets -- must be locked from the inside before push off from the gate.
"So for me there's only a few scenarios," said Paul Yap, an aviation lecturer at Temasek Polytechnic in Singapore.
"First the people involved in the deliberate actions are the pilots, one of them or both of them in cahoots.
"Then we have a scenario where terrorists make the pilots change course and switch off the transponders under duress, maybe threatening to kill passengers," Yap said.
The transponder of MH370 was switched off around the time analysts said it would have reached its cruising altitude, when pilots often emerge to take a bathroom or coffee break.
The hijackers of the four planes used in the 9/11 attacks turned off the transponders of three of the jets.
- Flight crew under scrutiny -
It was not clear if police have yet searched the homes of the other crew on Flight MH370, including that of First Officer Fariq, 27. His record and personal life have already come under scrutiny.
An Australian television report broadcast an interview with a young South African woman who said Fariq and another pilot colleague invited them into the cockpit of a flight he co-piloted from Phuket, Thailand to Kuala Lumpur in 2011.
Since 9/11, passengers have been prohibited from entering cockpits during a flight. Malaysia Airlines has said it was "shocked" by the report, but that it could not verify the claims.
The son of a high-ranking official in the public works department of a Malaysian state, Fariq joined Malaysia Airlines when he was 20.
He is a mild-mannered "good boy" who regularly visited his neighbourhood mosque outside Kuala Lumpur, said the mosque's imam, or spiritual leader.
The far more seasoned Zaharie joined MAS in 1981 and had logged 18,365 hours of flying time.
Malaysian media reports quoted colleagues calling Zaharie a "superb pilot", who also served as an examiner, authorised by the Malaysian Civil Aviation Department, to conduct simulator tests for pilots.
The whole passenger manifest is likely to be re-examined.
If hijackers are suspected, then the glare of suspicion will fall again on two passengers who boarded with stolen EU passports.
Interpol had identified the two men as Iranians: Seyed Mohammed Reza Delavar, who used a stolen Italian passport, and Pouria Nourmohammadi, who used an Austrian one.
Both passports had been stolen in Thailand.
Interpol chief Ronald Noble said last Tuesday that the men were thought to be illegal immigrants who had travelled from Doha to Kuala Lumpur in a round-about bid to reach Europe.
- Is hijacking a possibility? -
Interpol's information suggested the pair were "probably not terrorists", Noble said at the time.
Adam Dolnik, a professor of terrorism studies at the University of Wollongong in Australia, said he still doubted that organised terrorism was behind the Malaysian plane mystery.
While a group like Al-Qaeda "would love to bring down an airliner", a Malaysia Airlines plane made little sense as a target and the stolen passports had an "amateurish" element, Dolnik said.
"Terrorists don't do (hijackings), because the chances of success have gone down," he said, citing the challenge of bringing weapons onto a plane and subduing other passengers.
There has been no indication yet of any possible terrorist involvement.
But some academics suggest the theory requires further consideration.
"Investigations should focus on criminal and terrorist motives," said Rohan Gunaratna, a terrorism expert at Singapore's Nanyang Technological University.
"It is likely that the aircraft was hijacked by a team knowledgeable about airport and aircraft security. It is likely they are supported by a competent team from the ground."
Malaysia has not been the target of any notable terror attacks. But terror analysts say it is home to several individuals alleged to be operatives of militant Islamic groups such as the Al-Qaeda linked Jemaah Islamiyah.
Most of the passengers on the Beijing-bound Malaysia Airlines flight were Chinese nationals.
China is grappling with simmering anger among its Muslim ethnic Uighur minority in the country's remote far west, many of whom openly complain of Chinese repression.
It has blamed Uighur separatists for a string of violent incidents including a coordinated knife attack in the southwestern city of Kunming on March 1 that left 29 people dead.
Malaysia has deported at least 17 Uighur Muslims who were travelling on fake passports back to China since 2011.
London-based David Kaminski-Morrow, air transport editor for Flight International, warned of the danger of rushing to conclusions following Najib's announcement on Saturday.
"The new evidence is consistent with deliberate action, but it's still only a small amount of data -- certainly not a complete picture -- and therefore it's still premature to label the event formally as a hijack," Kaminski-Morrow said.