The missing Malaysia Airlines plane was "thrown around like a fighter jet" just after it lost contact with authorities in a bid to dodge radar, Malaysian military investigators believe.
Flight MH370, which disappeared more than a month ago en route to Beijing, is thought to have climbed to heights of 13,700m - 3000m above its normal altitude - before plummeting to just below 1500m.
The new lead in the investigation comes as the methodical search being carried out in the Indian Ocean continues amid fears that the jet's black box may have run out of battery.
The drastic manoeuvres which must have been taken for the plane's alitutude to change so suddenly suggest that the plane was deliberately trying to avoid radar signals and disappear, a source said.
Speaking to the Sunday Times, the source said: "It was being flown very low at very high speed. And it was being flown to avoid radar."
The new theory comes as the painstaking search for any sign of the jet - which was carrying 239 people - continues in the Indian Ocean.
A member of Kechara Buddhist organisation offers prayers for passengers onboard the missing Malaysia Airlines Flight 370. Photo / AP
Four strong underwater signals were picked up last week - hoped to be from the black box - but nothing new has been detected for five days.
Once officials are confident that no more sounds will be heard, a robotic submersible will be sent down to slowly scour for wreckage.
"We're now into day 37 of this tragedy," said aviation expert Geoffrey Thomas. "The battery life on the beacons is supposed to last 30 days. We're hoping it might last 40 days. However, it's been four or five days since the last strong pings.
"What they're hoping for is to get one more, maybe two more pings so they can do a triangulation of the sounds and try and narrow the (search) area."
A Royal Navy vessel - the HMS Echo - has now arrived to join the international hunt and is working round the clock to locate any signs of wreckage.
Survey ship HMS Echo is in the southern Indian Ocean helping in the underwater search for the flight recorder from the missing jet.
Hydrographic survey ship HMS Echo was diverted from work in the Indian Ocean to help in the hunt, and arrived on Thursday in the area of the southern Indian Ocean where "pings" thought to be from the missing plane had been detected.
Australian prime minister Tony Abbott said on Friday that crews had significantly narrowed down the search area in the hunt for signals which authorities are confident are from the missing jet.
But he said the signal from the Boeing 777's flight data and cockpit voice recorders is fading - batteries powering their locator beacons last only about a month and it has been more than a month since the plane disappeared.
Ocean Shield, an Australian ship towing a U.S. Navy device to detect signals from the beacons, first picked up two underwater sounds consistent with the "pings" last Saturday, followed by two more in the same general area on Tuesday.
Searchers are trying to pinpoint the location of the source of the signals so they can send down a robotic submersible to look for wreckage and the flight recorders.
HMS Echo, whose specialist equipment has been specially adapted to pick up sonar pings from the jet's missing black box, is supporting Ocean Shield by understanding the signals it is picking up, its commanding officer said.
Captain Flt. Lt. Tim McAlevey of the Royal New Zealand Air Force flies a P-3 Orion in search for the missing Malaysia Airlines Flight 370. Photo / AP
Speaking from the ship today, Commander Phillip Newell said they are working in conditions after inclement weather caused difficulties interpreting signals picked up underwater.
He said: "It's been challenging. Over the last couple of days we have been conducting oceanographic observations to support Ocean Shield.
"The key thing is to help their understanding of what is going on, and how it is affecting the ocean column."
He said there had been some difficult weather conditions which had "messed up" the water column, making it hard to understand some signals being picked up.
"The key challenge is to try and refine all of the observations they are making. They are doing that at the moment but it's challenging.
"Looking out of the window right now, what we are seeing is Ocean Shield to the south of us conducting further observations.
"She is trying to get in a position so she can then observe on the seabed, and then through the water column, the pings from the black box, which involves physically moving the ship."
He said another challenge is directing Australian navy P-3 Orion aircraft which drop sound-locating buoys, each dangling a hydrophone listening device about 1,000 feet below the surface, into the water.
Cdr Newell said he and his crew are conscious of the importance of their task and need to refine the search area while the black box's signal can still be detected.
Photo / AP
"At this stage it's a challenge trying to refine this position so that when they put a submersible into the water they will be in a position where they can identify what is on the seabed correctly.
"We have got to give them the best advantage we can and within that they will get to the position where they have the best refined position that they can search."
He added: "In anything like this I am very conscious, I have 20 years experience of trying to find things on the seabed, it's pretty much my day job.
"I have a brilliant team, young, bright and enthusiastic and we are working 24/7 to cover the sea bed and observe on the surface.
"There's a sense that we are playing an important part in this role and we are keen to get it right.
"In terms of purpose, it's key to make sure that we detect anything that can help in the investigation."
Before arriving in the latest search area HMS Echo had already searched 15,600 square kilometres of ocean - an area 10 times the size of Greater London - 1,600 kilometres north-west of Perth with Chinese vessels after sensors picked up a possible signal on April 5.
The Plymouth-based ship was gathering data on her way from Oman to the Seychelles when she was diverted to join the international search for the Malaysia Airlines plane.
Apart from a 12-hour stop in the Maldives to take on supplies and change some of her crew, the survey ship has now been at sea continuously for six weeks.
Cdr Newell said: "We are a ship that's designed and built to operate for long periods at sea. We can carry provisions for 60 days at sea, and fuel to go pretty much halfway round the world."
Hw said the ship would carry on helping with the search, providing as much support as it could to Ocean Shield until its tasking is reviewed later in the month.
The search is also being helped by nuclear submarine HMS Tireless.
- Daily Mail