(CNN) -- "All right, good night."
Those are the last words heard from the cockpit of Malaysia Airlines Flight 370.
Who said them? Was it the captain or his first mate? Or someone else in the cockpit with them?
Officials in Washington suspect it was either of the pilots -- and that one or both was involved in MH370's disappearance on March 8.
And according to the Malaysian Prime Minister's account of events, by the time those words were spoken, someone had likely already taken steps to alter the flight's path -- intentionally.
Malaysian investigators are not ruling out a hijacking by other actors. But they have searched the homes of the pilot and co-pilot.
Information from international and Malaysian officials indicate that the Boeing 777-200ER passenger jet may have flown for hours after that last voice contact with the pilots.
The duty of all pilots is to aviate, navigate and communicate, in that order, an aviation expert has told CNN.
Someone may have kept aviating, but either they couldn't -- or wouldn't -- communicate.
This is what we know about the 53-year-old pilot captain and his 27-year-old first mate.
Capt. Zaharie Ahmad Shah
Police had been outside his Malaysia home every day since the plane vanished, a source told CNN. But had not gone inside until recently.
When they did, they probably found a flight simulator there. In a YouTube video he apparently posted, Zaharie can be seen sitting in front of one.
And in a German online forum for simulator enthusiasts, X-Sim.de, there is a post from November 2012 in his name that says he built it himself.
"About a month ago I finish assembly of FSX and FS9 with 6 monitors." The message was signed Capt. Zaharie Ahmad Shah BOEING 777 MALAYSIA AIRLINES.
FSX and FS9 are over-the-counter flight simulator games made by Microsoft.
On Friday, the CEO of Malaysia Airlines said that everyone is allowed to pursue their hobbies.
Zaharie, a pilot with 18,365 flight hours under his belt, is reportedly also a flight instructor.
On the same YouTube channel, Zaharie gives workman's tips on tinkering with a refrigerator and an air conditioner.
CNN cannot verify the authenticity of the social media posts.
1st Officer Fariq Ab Hamid
CNN's aviation correspondent Richard Quest once visited MH370's 1st Officer Fariq Ab Hamid in a Malaysia Airlines cockpit, when he was training. Quest watched him land the plane under supervision of a senior pilot in February.
The captain described Fariq's landing as textbook perfect.
Fariq joined Malaysia Airlines in 2007. He has 2,763 flying hours behind him and was transitioning to the Boeing 777-200 after finishing training in a flight simulator.
As with Zaharie, not much is known to the public about Fariq. But Quest was not the only guest who had joined him in the cockpit.
Passenger Jonti Roos got an invitation to check out the cockpit during a flight from Thailand to Malaysia -- one that Fariq was flying with another pilot.
She took photos and said Farid and his colleague smoked in the cockpit. After MH 370's disappearance, she reported her experience to journalists.
Malaysia Airlines was aghast. "We are shocked by these allegations," the airline said.
Such a practice would be illegal on U.S. carriers after the September 11, 2001, terror attacks, but not necessarily so on international ones, Quest said.
Exploring the possibilities
Does Roos' story open up the possibility that a third or fourth person could have joined Zaharie and Fariq in the cockpit?
Like the exact whereabouts of Flight 370, that's yet unknown. But investigators believe that somebody must have done something.
Not long after the flight took off from Kuala Lumpur communications systems were disabled, the plane's transponder was turned off, Malaysian Prime Minister Najib Razak said Saturday.
That last device is situated between the pilots and can be shut off with a twist of the wrist. For a pilot to turn it off would seem reckless because the information it transmits gives the plane vital protection. It helps people on the ground locate the plane.
Someone would have to know how to do it and also know the plane would lose that protection.
Then someone in the cockpit said good night.
And the apparent lack of visibility on radar? "Airline pilots are not trained for radar avoidance," said aviation expert Keith Wolzinger, a former 777 pilot. They like to stay on the radar, because -- again -- it protects their plane.
Only military pilots, he said, are usually keen on avoiding radar.
The father of a passenger on the missing plane is hoping for an outcome that would sound shocking under normal circumstances.
"I hope the plane was hijacked, because then, at least, there is hope," Li from Hebei Province said. He did not give his full name.
Li is waiting at a Beijing hotel with dozens of other passengers' family members awaiting word on the fate of their loved ones.
"But if the worst happened then I will have no meaning in my life. This is my only son," Li said.
As he walked away, he bent his head and cried.
CNN's Pauline Chiou contributed from Beijing