Two minutes after the co-pilot of MH370 said "all right, goodnight", air traffic control lost sight of the Malaysia Airlines Boeing 777-200 aircraft.
This was when the plane's transponder - a device in the aircraft which transmits data on location and altitude - was believed to have been switched off. The time was 1.21am.
Any B-777 pilot or someone who knows about aircraft control systems can turn off the transponder simply by flicking an on/off switch, say pilots.
The plane is also equipped with the Aircraft and Communications Addressing and Reporting System (ACARS), which uses radio or satellite signals to send data that monitor engines and other equipment.
Like the transponder, it enables the aircraft to be tracked while in flight.
To stop the ACARS from transmitting data by radio, a series of cockpit switches would have to be flicked in sequence, and some computer input is needed as well.
The tricky part is to turn off satellite transmission, which was not done in MH370's case. To deactivate it, one would have to get to an electrical panel that is not easily accessible.
It explains why the authorities later said the aircraft continued to emit blips for several more hours after it broke contact with civilian air traffic control.
The last radio transmission from ACARS was 1.07am, 12 minutes before First Officer Fariq Abdul Hamid, 27, signed off from the Malaysian air traffic control.
The next transmission should have happened at 1.37am, but did not. The authorities believe the system was deliberately shut down some time between 1.07am and 1.37am.
Was this before or after the first officer said goodbye? If it was before, he would be high on the list of suspects responsible for the deliberate diversion of the flight, as it suggests he deliberately did not report that the system was not working.
Another possibility would be that the plane had been hijacked, and the pilots were forced to divert it, or the hijackers had taken over the piloting of the plane.
The recorded voice, which sounded "calm", will be analysed to verify if Mr Fariq was under stress when he said those words.
Sitting next to him in the cockpit of the plane that had left Kuala Lumpur at 12.41am on March 8 for Beijing was Captain Zaharie Ahmad Shah, 53.
He is under investigation too, along with 10 cabin crew and 227 passengers on board the plane.
If the plane had been hijacked, it would have happened before the transponder was turned off at 1.21am. By then, it was 40 minutes into the flight. At this stage, the aircraft should have reached cruising altitude of more than 30,000 feet (9,144m).
Typically, passengers are free to walk around the cabin once the plane reaches about 10,000 feet. Pilots are also free to leave the cockpit. Depending on factors like weather, it takes about 15 to 20 minutes to reach that altitude.
If the MH370 pilots were not responsible for diverting the plane, the question is, how did someone else get into the cockpit?
Since the Sept 11, 2001, terrorist attacks, airlines have beefed up cockpit security.
All doors are locked from the inside, and there is a camera in the cockpit so pilots can monitor what goes on in the area just outside and around the cockpit door.
Only cabin crew are allowed into the cockpit, only to serve meals or if there is an emergency.
To enter, they must press a call button. The pilots then scan the area outside before they unlock the door.
If there is an emergency and cabin crew members have to enter the cockpit, they can do so by keying in an emergency code. Pilots have about 30 seconds to override this and keep the door locked.
