Officials in Malaysia say they believe the co-pilot of missing flight MH370 spoke the last words to ground controllers before it vanished.

Investigators are looking into the possibility that the aircraft's crew were involved in its disappearance.

The search for the plane has extended into two vast air corridors.

Twenty-six countries have been asked to help find the jet, which went missing over a week ago with 239 people on board.

Malaysia says the plane was intentionally diverted and could have flown on either a northern or southern arc from its last known position.

Ahmad Jauhari Yahy, chief executive of Malaysia Airlines, told a news conference on Monday that initial investigations had indicated that co-pilot Fariq Abdul Hamid had calmly said "all right, good night" shortly before the plane disappeared.

However it is not clear whether the last words came before or after one of the plane's tracking devices was switched off. Officials believe the communications systems were deliberately disabled.

'Switched off'

Police have searched the homes of Captain Zaharie Shah, 53, and Mr Fariq, 27. A flight simulator taken from the captain's home was being reassembled and examined at police headquarters, officials said.

Investigators are also looking at passengers, engineers and other ground staff who may have had contact with the aircraft before take-off.

The plane left Kuala Lumpur for Beijing at 00:40 local time (16:40 GMT) on 8 March.

Officials say the sign-off to air traffic controllers came at 01:19 as it left Malaysian airspace.

The last transmission from the plane's Aircraft and Communications Addressing and Reporting System (ACARS) was received at 01:07.

"We don't know when the ACARS was switched off after that," Mr Ahmad Jauhari said. "It was supposed to transmit 30 minutes from there, but that transmission did not come through."

The plane disappeared from air traffic controllers' screens at 01:21, when it was over the South China Sea.

Malaysia's Prime Minister Najib Razak said on Saturday that satellite and radar evidence showed it had changed course and could have continued flying for a further seven hours.

'Anguish'

Searches have started in two air corridors - one stretching from the border of Kazakhstan and Turkmenistan to northern Thailand, and another from Indonesia to the Indian Ocean.

"Today, I can confirm that search and rescue operations in the northern and southern corridors have already begun," Malaysian Transport Minister Hishammuddin Hussein told a news conference on Monday.

Australian Prime Minister Tony Abbott said on Monday he had agreed to take the lead in scouring the southern Indian Ocean for the "ill-fated aircraft".

Mr Hussein said: "Over the past two days, we have been recalibrating the search for MH370."

He also responded to speculation that the Malaysian authorities had withheld information, saying they had been open throughout the investigation.

"For the families, I understand that every day prolongs the anguish," he said. "I understand because Malaysia, too, is missing its sons and daughters.

"Our priority has always been to find the aircraft. We would not withhold any information that could help. But we also have a responsibility not to release information until it has been verified by the international investigations team."

Two-thirds of the passengers were from China and the country's state media has been criticising the Malaysian operation.

Chinese Premier Li Keqiang in a phone call asked the Malaysian PM to provide more detailed information about the missing flight "in a timely, accurate and comprehensive manner", state news agency Xinhua reported on Monday.

Relatives of the missing passengers and crew members have now been waiting desperately for updates for 10 days since the aircraft disappeared.

One woman, whose older sister was making her first plane journey, told the BBC she thought the Malaysian authorities were holding information back,

"Why don't they tell us the truth?" she asked. "We are so anxious just waiting here."

Meanwhile, three French officials involved in the search for Air France flight 447 from Rio de Janeiro to Paris, which crashed in 2009, have arrived in Malaysia to offer their expertise.