KUALA LUMPUR, Malaysia — The search for the missing Malaysian jet pushed deep into the northern and southern hemispheres Monday as Australia scoured the southern Indian Ocean and Kazakhstan — more than 10,000 kilometers (6,000 miles) to the northwest — answered Malaysia’s call for help in the unprecedented hunt.

French investigators arriving in Kuala Lumpur to lend expertise from the two-year search for an Air France jet that crashed into the Atlantic Ocean in 2009 said they were able to rely on distress signals. But that vital tool is missing in the Malaysia Airlines mystery because flight 370’s communications were deliberately severed ahead of its disappearance more than a week ago, investigators say.

“It’s very different from the Air France case. The Malaysian situation is much more difficult,” said Jean Paul Troadec, a special adviser to France’s aviation accident investigation bureau.

Malaysian authorities say the jet carrying 239 people was intentionally diverted from its flight path during an overnight flight from Kuala Lumpur to Beijing on March 8. Suspicion has fallen on the pilots, although Malaysian officials have said they are seeking background checks on everyone aboard the flight.

Malaysian police confiscated a flight simulator from the pilot’s home on Saturday and also visited the home of the co-pilot in what Malaysian police chief Khalid Abu Bakar initially said was the first police visits to those homes. But the government issued a statement Monday contradicting that account by saying police first visited the pilots’ homes as early as March 9, the day after the flight.

Investigators haven’t ruled out hijacking, sabotage, pilot suicide or mass murder, and they are checking the backgrounds of all 227 passengers and 12 crew members, as well as the ground crew, to see if links to terrorists, personal problems or psychological issues could be factors.

However, Malaysian Defense Minister Hishammuddin Hussein said at a news conference Monday that finding the plane was still the main focus, and he did not rule out finding it intact.

“The fact that there was no distress signal, no ransom notes, no parties claiming responsibility, there is always hope,” Hishammuddin said.

Malaysian Airlines CEO Ahmad Jauhari Yahya said an initial investigation indicated that the co-pilot, Fariq Abdul Hamid, spoke the fight’s last words — “All right, good night” — to ground controllers.

Malaysian officials earlier said those words came after one of the jetliner’s data communications systems — the Aircraft Communications Addressing and Reporting System — had been switched off, sharpening suspicion that one or both of the pilots may have been involved in the plane’s disappearance.

However, Ahmad said Monday that while the last data transmission from ACARS — which gives plane performance and maintenance information — came before that, it was still unclear at what point the system was switched off. That opened the possibility that both ACARS and the plane’s transponders — which make the plane visible to civilian air traffic controllers — were severed later and at about the same time.

Malaysia’s government in the meantime sent out diplomatic cables to all countries in the search area, seeking their help in providing planes and ships for the search, as well as to ask for any radar data that might help narrow the task.

Some 26 countries are involved in the search, which initially focused on seas on either side of peninsular Malaysia, in the South China Sea and the Strait of Malacca.

Over the weekend, however, Malaysian Prime Minister Najib Razak announced that investigators determined that a satellite picked up a faint signal from the aircraft about 7  1/2 hours after takeoff. The signal indicated that the plane would have been somewhere on a vast arc stretching from Kazakhstan down to the southern reaches of the Indian Ocean.

Hishammuddin said Monday that searches in both the northern and southern stretches of the arc had begun, with countries from Australia up north to China and west to Kazakhstan joining the hunt.

Had the plane gone northwest to Central Asia, it would have crossed over countries with busy airspace, and some experts believe the person in control of the aircraft would more likely have chosen to go south. However, authorities are not ruling out the northern corridor and are eager for radar data that might confirm or rule out that path.

The northern search corridor crosses through countries including China, India and Pakistan — all of which have indicated they have seen no sign of the plane.

An official with the Chinese civil aviation authority said the missing plane did not enter Chinese airspace, but the Chinese Defense Ministry and Foreign Ministry didn’t immediately respond to questions on radar information.

China, where two thirds of the passengers were from, is providing several planes and 21 satellites for the search, Premier Li Keqiang said in a statement.

“Factors involved in the incident continue to multiply, the area of search and rescue continues to broaden, and the level of difficulty increases, but as long as there is one thread of hope, we will continue an all-out effort,” Li said.

Indonesian officials have said the plane did not cross their territory, based on radar data. Air force spokesman Rear Marshall Hadi Tjahjanto said Monday his country’s search efforts were focusing on waters west of Sumatra in the Indian Ocean.

Australian Prime Minister Tony Abbott told Parliament that he agreed to take the lead in scouring the southern Indian Ocean for the aircraft during a conversation Monday with Malaysia’s leader.

“Australia will do its duty in this matter,” Abbott told Parliament. “We will do our duty to the families of the 239 people on that aircraft who are still absolutely devastated by their absence, and who are still profoundly, profoundly saddened by this as yet unfathomed mystery.”

Two Australian Orion maritime planes that have been searching for the past week headed Monday to the southern Indian Ocean, with two more to join them in the coming 24 hours, Abbott said. New Zealand and U.S. planes also will join that team.

The southern Indian Ocean is the world’s third-deepest and one of the most remote stretches of water in the world, with little radar coverage.