* Air force chief denies saying plane tracked to Malacca Strait
* Source had said radar suggested plane turned back and flew some distance
* Hijack, sabotage, mechanical failure also being investigated
* Still no sign of missing plane, search in fifth day
* 227 passengers and 12 crew aboard lost flight
By Stuart Grudgings
KUALA LUMPUR, March 12 (Reuters) - Malaysia's air force chief has denied saying military radar tracked a missing Malaysia Airlines jetliner over the Strait of Malacca, adding to the mystery surrounding the fate of flight MH370, which vanished on Saturday with 239 people aboard.
A massive air and sea search now in its fifth day has failed to find any trace of the Boeing 777, and the last 24 hours have seen conflicting statements and reports over what may have happened after it lost contact with air traffic controllers.
Malaysia's Berita Harian newspaper on Tuesday quoted Air Force chief Rodzali Daud as saying the plane was last detected by military radar at the northern end of the Strait of Malacca at 2.40 a.m. on Saturday, hundreds of kilometres off course.
"I wish to state that I did not make any such statements," Rodzali said in a statement on Wednesday. The air force chief said he had merely repeated that military radar tracking suggested the plane might have turned back.
A senior military officer who had been briefed on the investigation told Reuters on Tuesday that the aircraft had made a detour to the west after communications with civilian authorities ended.
"It changed course after Kota Bharu and took a lower altitude. It made it into the Malacca Strait," the officer said.
Malaysian authorities have said previously that flight MH370 disappeared around 1.30 a.m., roughly midway between Malaysia's east coast town of Kota Bharu and southern Vietnam, about an hour after it took off from Kuala Lumpur bound for Beijing.
The Strait of Malacca, one of the world's busiest shipping channels, runs along Malaysia's west coast, while Kota Bharu is on the northeast coast.
After the comments from the officer, a non-military source familiar with the investigations said the reported detour was one of several theories and was being checked.
If the plane had made such a detour it would undermine the theory that it suffered a sudden, catastrophic mechanical failure, as it would mean it flew at least 500 km (350 miles) after its last contact with air traffic control.
A spokesman for the Malaysian prime minister's office said on Wednesday he had not been informed by the military of evidence showing the plane had recrossed the Malay Peninsula to reach the Malacca Strait.
"The people I checked with were not aware of that," spokesman Tengku Sariffuddin Tengku Ahmad told Reuters.
HUGE INTERNATIONAL SEARCH
A huge international search operation has been mostly focused on the shallow waters of the Gulf of Thailand off Malaysia's east coast, although the Strait of Malacca has been included since Sunday.
Navy ships, military aircraft, helicopters, coast guard and civilian vessels from 10 nations have criss-crossed the seas off both coasts of Malaysia without success.
In the absence of any concrete evidence to explain the plane's disappearance, authorities have not ruled out anything. Police have said they were investigating whether any passengers or crew on the plane had personal or psychological problems that might shed light on the mystery, along with the possibility of a hijacking, sabotage or mechanical failure.
"Maybe somebody on the flight has bought a huge sum of insurance, who wants family to gain from it or somebody who has owed somebody so much money, you know, we are looking at all possibilities," Malaysian police chief Khalid Abu Bakar said at a news conference on Tuesday.
The airline said it was taking seriously a report by a South African woman who said the co-pilot of the missing plane had invited her and a female travelling companion to sit in the cockpit during a flight two years ago, in an apparent breach of security.
"Malaysia Airlines has become aware of the allegations being made against First Officer Fariq Ab Hamid which we take very seriously. We are shocked by these allegations. We have not been able to confirm the validity of the pictures and videos of the alleged incident," the airline said in a statement.
The woman, Jonti Roos, said in an interview with Australia's Channel Nine TV that she and her friend were invited to fly in the cockpit by Fariq and the pilot between Phuket, Thailand, and Kuala Lumpur in December 2011. The TV channel showed pictures of the four apparently in a plane's cockpit.
The Boeing 777 has one of the best safety records of any commercial aircraft in service. Its only previous fatal crash came on July 6 last year when Asiana Airlines Flight 214 struck a seawall on landing in San Francisco, killing three people.
U.S. planemaker Boeing has declined to comment beyond a brief statement saying it was monitoring the situation.