Story highlights 6th underwater drone mission finishes; 7th such trip is underway

Agency: Up to 11 military planes, 12 ships to take part in Saturday search

Flight 370 is said to have deviated from flight plan while inside Vietnamese airspace

There's still no sign of the Boeing 777, now 6 weeks after it went missing

Malaysia Airlines Flight 370 drastically changed course then soared to near its peak altitude, a senior Malaysian aviation source said -- adding yet another wrinkle to the enigma of the plane's last flight.

Before disappearing from radar screens on March 8, the commercial airliner deviated from its planned route from Kuala Lumpur to Beijing by turning leftward over water while it was still inside Vietnamese airspace, the Malaysian aviation source told CNN's Nic Robertson.

The aircraft then climbed to 39,000 feet, just short of the Boeing 777-200ER's 41,000-foot safe operating limit, and maintained that altitude for about 20 minutes over the Malaysian Peninsula before beginning to descend, the source said.

Why? That and so much else -- including where the plane is now -- remains a mystery. Investigators have been trying for weeks to piece together bits of information trying to get the answers being demanded by relatives of the 239 people aboard the plane, not to mention millions more around the world who have been captivated by this ordeal.

In addition to this newly revealed development, investigators have determined that the missing jet was equipped with four emergency locator transmitters, or ELTs, which are designed to transmit a plane's location to an emergency satellite when triggered by a crash or by contact with water, the source added.

The ELTs were at the plane's front door, its rear door, in the fuselage and in the cockpit, said the source, who was puzzled over why they appear either not to have activated or, if they did activate, why they were not picked up by the satellite.

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Photos: The search for MH370 Photos: The search for MH370 Two years after Malaysia Airlines Flight 370 went missing, a relative of one of the passengers burns incense in Beijing on March 8, 2016. Flight 370 vanished on March 8, 2014, as it flew from Kuala Lumpur, Malaysia, to Beijing. There were 239 people on board. Hide Caption 1 of 43 Photos: The search for MH370 On July 29, police carry a piece of debris on Reunion Island, a French territory in the Indian Ocean. A week later, authorities confirmed that the debris was from the missing flight. Hide Caption 2 of 43 Photos: The search for MH370 Staff members with the Australian Transport Safety Bureau examine a piece of aircraft debris at their laboratory in Canberra, Australia, on July 20. The flap was found in June by residents on Pemba Island off the coast of Tanzania, and officials had said it was highly likely to have come from Flight 370. Experts at the Australian Transport Safety Bureau, which is heading up the search for the plane, confirmed that the part was indeed from the missing aircraft. Hide Caption 3 of 43 Photos: The search for MH370 In late February, American tourist Blaine Gibson found a piece of plane debris off Mozambique, a discovery that renewed hope of solving the mystery of the missing flight. The piece measured 35 inches by 22 inches. A U.S. official said it was likely the wreckage came from a Boeing 777, which MH370 was. Hide Caption 4 of 43 Photos: The search for MH370 Relatives of the flight's passengers console each other outside the Malaysia Airlines office in Subang, Malaysia, on February 12, 2015. Protesters had demanded that the airline withdraw the statement that all 239 people aboard the plane were dead. Hide Caption 5 of 43 Photos: The search for MH370 A police officer watches a couple cry outside the airline's office building in Beijing after officials refused to meet with them on June 11, 2014. The couple's son was on the plane. Hide Caption 6 of 43 Photos: The search for MH370 Members of the media scramble to speak with Azharuddin Abdul Rahman, director general of Malaysia's Civil Aviation Department, at a hotel in Kuala Lumpur, Malaysia, on May 27, 2014. Data from communications between satellites and the missing flight was released the day before, more than two months after relatives of passengers said they requested it be made public. Hide Caption 7 of 43 Photos: The search for MH370 Operators aboard the Australian ship Ocean Shield move Bluefin-21, the U.S. Navy's autonomous underwater vehicle, into position to search for the jet on April 14, 2014. Hide Caption 8 of 43 Photos: The search for MH370 A member of the Royal New Zealand Air Force looks out of a window while searching for debris off the coast of western Australia on April 13, 2014. Hide Caption 9 of 43 Photos: The search for MH370 The HMS Echo, a vessel with the British Roya; Navy, moves through the waters of the southern Indian Ocean on April 12, 2014. Hide Caption 10 of 43 Photos: The search for MH370 A Royal Australian Air Force AP-3C Orion, on a mission to drop sonar buoys to assist in the search, flies past the Australian vessel Ocean Shield on April 9, 2014. Hide Caption 11 of 43 Photos: The search for MH370 A relative of a missing passenger cries at a vigil in Beijing on April 8, 2014. Hide Caption 12 of 43 Photos: The search for MH370 Australian Defense Force divers scan the water for debris in the southern Indian Ocean on April 7, 2014. Hide Caption 13 of 43 Photos: The search for MH370 A towed pinger locator is readied to be deployed off the deck of the Australian vessel Ocean Shield on April 7, 2014. Hide Caption 14 of 43 Photos: The search for MH370 A member of the Royal New Zealand Air Force looks at a flare in the Indian Ocean during search operations on April 4, 2014. Hide Caption 15 of 43 Photos: The search for MH370 On March 30, 2014, a woman in Kuala Lumpur prepares for an event in honor of those aboard Flight 370. Hide Caption 16 of 43 Photos: The search for MH370 The sole representative for the families of Flight 370 passengers leaves a conference at a Beijing hotel on March 28, 2014, after other relatives left en masse to protest the Malaysian government's response to their questions. Hide Caption 17 of 43 Photos: The search for MH370 A member of the Royal Australian Air Force is silhouetted against the southern Indian Ocean during the search for the missing jet on March 27, 2014. Hide Caption 18 of 43 Photos: The search for MH370 Flight Lt. Jayson Nichols looks at a map aboard a Royal Australian Air Force aircraft during a search on March 27, 2014. Hide Caption 19 of 43 Photos: The search for MH370 People in Kuala Lumpur light candles during a ceremony held for the missing flight's passengers on March 27, 2014. Hide Caption 20 of 43 Photos: The search for MH370 Malaysian Prime Minister Najib Razak, center, delivers a statement about the flight on March 24, 2014. Razak's announcement came after the airline sent a text message to relatives saying it "deeply regrets that we have to assume beyond any reasonable doubt that MH 370 has been lost and that none of those onboard survived." Hide Caption 21 of 43 Photos: The search for MH370 Grieving relatives of missing passengers leave a hotel in Beijing on March 24, 2014. Hide Caption 22 of 43 Photos: The search for MH370 A passenger views a weather map in the departures terminal of Kuala Lumpur International Airport on March 22, 2014. Hide Caption 23 of 43 Photos: The search for MH370 A Chinese satellite captured this image, released on March 22, 2014, of a floating object in the Indian Ocean, according to China's State Administration of Science. It was a possible lead in the search for the missing plane. Surveillance planes were looking for two objects spotted by satellite imagery in remote, treacherous waters more than 1,400 miles from the west coast of Australia. Hide Caption 24 of 43 Photos: The search for MH370 Satellite imagery provided by the Australian Maritime Safety Authority on March 20, 2014, showed debris in the southern Indian Ocean that could have been from Flight 370. The announcement by Australian officials raised hopes of a breakthrough in the frustrating search. Hide Caption 25 of 43 Photos: The search for MH370 Another satellite shot provided by the Australian Maritime Safety Authority shows possible debris from the flight. Hide Caption 26 of 43 Photos: The search for MH370 A distraught relative of a missing passenger breaks down while talking to reporters at Kuala Lumpur International Airport on March 19, 2014. Hide Caption 27 of 43 Photos: The search for MH370 On March 18, 2014, a relative of a missing passenger tells reporters in Beijing about a hunger strike to protest authorities' handling of information about the missing jet. Hide Caption 28 of 43 Photos: The search for MH370 U.S. Navy crew members assist in search-and-rescue operations in the Indian Ocean on March 16, 2014. Hide Caption 29 of 43 Photos: The search for MH370 Members of the Chinese navy continue search operations on March 13, 2014. After starting in the sea between Malaysia and Vietnam, the plane's last confirmed location, search efforts expanded west into the Indian Ocean. Hide Caption 30 of 43 Photos: The search for MH370 A Vietnamese military official looks out an aircraft window during search operations March 13, 2014. Hide Caption 31 of 43 Photos: The search for MH370 Malaysian air force members look for debris near Kuala Lumpur on March 13, 2014. Hide Caption 32 of 43 Photos: The search for MH370 Relatives of missing passengers wait for the latest news at a hotel in Beijing on March 12, 2014. Hide Caption 33 of 43 Photos: The search for MH370 A member of the Vietnamese air force checks a map while searching for the missing plane on March 11, 2014. Hide Caption 34 of 43 Photos: The search for MH370 A Vietnamese air force plane found traces of oil that authorities had suspected to be from the missing Malaysia Airlines plane, the Vietnamese government online newspaper reported on March 8, 2014. However, a sample from the slick showed it was bunker oil, typically used to power large cargo ships, Malaysia's state news agency, Bernama, reported on March 10, 2014. Hide Caption 35 of 43 Photos: The search for MH370 A U.S. Navy Seahawk helicopter lands aboard the USS Pinckney to change crews on March 9, 2014, before returning to search for the missing plane in the Gulf of Thailand. Hide Caption 36 of 43 Photos: The search for MH370 Buddhist monks at Kuala Lumpur International Airport offer a special prayer for the missing passengers on March 9, 2014. Hide Caption 37 of 43 Photos: The search for MH370 Members of a Chinese emergency response team board a rescue vessel at the port of Sanya in China's Hainan province on March 9, 2014. Hide Caption 38 of 43 Photos: The search for MH370 The rescue vessel sets out from Sanya in the South China Sea on March 9, 2014. Hide Caption 39 of 43 Photos: The search for MH370 Malaysian Prime Minister Najib Razak, center, arrives to meet family members of missing passengers at the reception center at Kuala Lumpur International Airport on March 8, 2014. Hide Caption 40 of 43 Photos: The search for MH370 A relative of two missing passengers reacts at their home in Kuala Lumpur on March 8, 2014. Hide Caption 41 of 43 Photos: The search for MH370 Chinese police at the Beijing airport stand beside the arrival board showing delayed Flight 370 in red on March 8, 2014. Hide Caption 42 of 43 Photos: The search for MH370 Malaysia Airlines Group CEO Ahmad Juahari Yahya, front, speaks during a news conference at a hotel in Sepang on March 8, 2014. "We deeply regret that we have lost all contacts" with the jet, he said. Hide Caption 43 of 43

Relatives of the 239 passengers and crew have raised questions about the ELTs with Malaysian authorities, suggesting there were at least three aboard the plane, including two portable units and one fixed device.

No comment from Malaysia Airlines

Malaysia Airlines has declined to answer CNN's questions about the ELTs and other matters pertaining to the flight, which vanished six weeks ago after taking off shortly after midnight from Kuala Lumpur, Malaysia.

The airline said it could not comment on "any questions that relate to information held by other authorities and/or fall under the jurisdiction of the ongoing investigation. ..."

Besides this probe, the plight has spurred an expansive, expensive search to find the aircraft.

That includes the dispatching of up to 11 military aircraft and 12 ships Saturday across three areas off Perth, Australia. They will cover about 20,000 square miles (50,000 square kilometers) and brave isolated showers, according to Australia's Joint Agency Coordination Centre.

There may not be much more of this, however. Officials have said searches from air and ships are probably nearing an end.

That doesn't surprise former U.S. National Transportation Safety Board Managing Director Peter Goelz, given the results so far. "There's a lot of resources being expended there; it's turned up nothing," he said.

But Goelz predicts the underwater phase of the search will continue for the six to eight weeks needed to cover the current search zone. If that turns up nothing, he predicted, towed array sonar probably would be used to search a wider zone.

"This is a very complex operation," ocean search specialist Rob McCallum said. "It's going to be a game of patience now."

As of early Saturday, the underwater drone scouring the bottom of the Indian Ocean had taken six trips looking for the missing jetliner with a seventh mission then underway.

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The Joint Agency Coordination Centre said the Bluefin-21 search has covered about 50 square miles. While the information gleamed from the sixth trip was still being analyzed, the first five didn't yield any breakthroughs.

Malaysia's acting transport minister, Hishammuddin Hussein tweeted Friday that authorities are looking at deploying more unmanned underwater probes.

Officials might consider searching along a large portion of sea highlighted by a partial digital "handshake" between the jetliner and an Inmarsat PLC satellite, said Martin Dolan, Australia's top transport official.

That arc of sea is more than 370 miles long and 30 miles wide.

A prolonged undersea search by private contractors could cost a "ballpark rough estimate" of $234 million, said Dolan.

Passengers' kin list questions

The continuing search efforts came as relatives of the people who were aboard the jetliner pressed for answers.

They have drawn up 26 questions that they want addressed by Malaysian officials, who are to meet with them next week in Beijing. Most of the Flight 370 passengers and crew were Chinese.

Among their questions: What's in the flight's log book? Can they review the jet's maintenance records? Can they listen to recordings of the Boeing 777 pilot's conversations with air traffic controllers just before contact was lost?

Hishammuddin has defended his government's handling of the operation and accused members of the news media of focusing on the Chinese families. He said relatives of passengers and crew from other nations represented have not had problems.

"The most difficult part of any investigation of this nature is having to deal with the families," he said.