By Niluksi Koswanage and Siva Govindasamy

KUALA LUMPUR (Reuters) - An investigation into the disappearance of a Malaysia Airlines jetliner is focusing more on a suspicion of foul play, as evidence suggests it was diverted hundreds of miles off course, sources familiar with the Malaysian probe said.

In a far more detailed description of military radar plotting than has been publicly revealed, two sources told Reuters an unidentified aircraft that investigators suspect was missing Flight MH370 appeared to be following a commonly used navigational route when it was last spotted early on Saturday, northwest of Malaysia.

That course - headed into the Andaman Sea and towards the Bay of Bengal in the Indian Ocean - could only have been set deliberately, either by flying the Boeing 777-200ER jet manually or by programming the auto-pilot.

A third investigative source said inquiries were focusing more on the theory that someone who knew how to fly a plane deliberately diverted the flight hundreds of miles off its scheduled course from Kuala Lumpur to Beijing.

"What we can say is we are looking at sabotage, with hijack still on the cards," said the source, a senior Malaysian police official.

GRAPHICS

- Navigation waypoints on route http://link.reuters.com/mug67v

- Expanded search area map http://link.reuters.com/rac57v

- Primary and secondary radar http://link.reuters.com/req57v

- How black boxes work http://link.reuters.com/wux57v

- MH370 and other incidents http://link.reuters.com/cyx57v

One of the most baffling mysteries in the history of modern aviation remains unsolved after nearly a week.

The latest radar evidence is consistent with the expansion of the search for the aircraft to the west of Malaysia.

There has been no trace of the plane nor any sign of wreckage as the navies and military aircraft of more than a dozen countries scour the seas across Southeast Asia.

Malaysian Transport Minister Hishammuddin Hussein said he could not confirm the last heading of the plane or if investigators were focusing on sabotage.

"A normal investigation becomes narrower with time ... as new information focuses the search, but this is not a normal investigation," he told a news conference. "In this case, the information has forced us to look further and further afield."

Investigators were still looking at "four or five" possibilities, including a diversion that was intentional or under duress, or an explosion, he said. Police would search the pilot's home if necessary and were still investigating all 239 passengers and crew on the plane, he added.

INDIAN OCEAN "BIGGEST CHALLENGE"

If the jetliner did stray into the Indian Ocean, a vast expanse with depths of more than 7,000 metres (23,000 feet), the task faced by searchers would become dramatically more difficult. Winds and currents could shift any surface debris tens of nautical miles within hours, dramatically widening the search area with each passing day.

"Ships alone are not going to get you that coverage, helicopters are barely going to make a dent in it and only a few countries fly P-3s (long-range search aircraft)," William Marks, spokesman for the U.S. Seventh Fleet, told Reuters.

"So this massive expanse of water space will be the biggest challenge."

The U.S. Navy was sending an advanced P-8A Poseidon plane to help search the Strait of Malacca, a busy sealane separating the Malay Peninsula from the Indonesian island of Sumatra. It had already deployed a Navy P-3 Orion aircraft to those waters.

U.S. defence officials told Reuters that the U.S. Navy guided-missile destroyer, USS Kidd, was heading to the Strait of Malacca, answering a request from the Malaysian government. The Kidd had been searching the areas south of the Gulf of Thailand, along with the destroyer USS Pinckney.

Satellites picked up faint electronic pulses from the aircraft after it went missing on Saturday, but the signals gave no immediate information about where the jet was heading and little else about its fate, two sources close to the investigation said on Thursday.

U.S. experts are still examining the data to see if any information about its last location could be extracted, a source close to the investigation told Reuters. Malaysia's civil aviation chief confirmed on Friday the government was working with U.S. investigators to establish if there was any satellite information that could help locate the airliner.

Story continues