Story highlights CNN aviation analyst says surface slick could be fluid leaking from underwater plane

Cell phone tower detected co-pilot's phone around time of disappearance, official says

An oil slick has been found 5.5 kilometers (3.4 miles) from where pings were detected

Underwater search vehicle comes back to surface early after exceeding depth limit

The phone of the first officer of Malaysia Airlines Flight 370 was on and made contact with a cell tower in Malaysia about the time the plane disappeared from radar, a U.S. official told CNN on Monday.

However, the U.S. official -- who cited information shared by Malaysian investigators -- said there was no evidence the first officer, Fariq Abdul Hamid, had tried to make a call.

The official told CNN's Pamela Brown on Monday that a cell-phone tower in Penang, Malaysia -- about 250 miles from where the flight's transponder last sent a signal -- detected the first officer's phone searching for service roughly 30 minutes after authorities believe the plane made a sharp turn westward.

The details do appear to reaffirm suggestions based on radar and satellite data that the plane was off course and was probably flying low enough to obtain a signal from a cell tower, the U.S. official said.

The revelation follows reporting over the weekend in a Malaysian newspaper that the first officer had tried to make a telephone call while the plane was in flight.

Asked Sunday by CNN about the newspaper report about a purported effort to make a call by the first officer, Malaysia's acting transport minister Hishammuddin Hussein said, "As far as I know, no, but as I said that would be in the realm of the police and the other international (authorities) and when the time comes that will be revealed. But I do not want to speculate on that at the moment."

U.S. officials familiar with the investigation told CNN they have been told that no other cell phones were picked up by the Penang tower.

Pilots are supposed to turn off their cell phones before pushing back from the gate.

"It would be very rare in my opinion to have someone with a cell phone on in the cockpit," safety analyst David Soucie said. "It's never supposed to be on at all. It's part of every check list of every airline I am familiar with."

When the plane first went missing, authorities said millions of cell phone records were searched, looking for evidence that calls had been made from the plane after it took off, but the search turned up nothing.

Underwater search is shortened

Efforts to find the missing plane and the 239 people aboard were focused beneath the choppy surface of the southern Indian Ocean on Monday as Australian authorities sent a U.S. Navy-contracted submersible diving toward the sea floor.

But after completing just six hours of searching for underwater debris, the autonomous underwater vehicle Bluefin-21 returned to the surface, according to the Joint Agency Coordination Centre in Perth, Australia.

It should have taken the probe and its operators 24 hours to map the first portion of the search area: 16 hours to map, four hours' travel time to just above the ocean floor and back, and four hours for analysts to examine the data gathered.

It is unclear how much of the area -- 5 kilometers by 8 kilometers (3.1 miles by 4.9 miles) -- the Bluefin scanned. It could take up to two months to scan the entire search area.

Officials said a built-in safety feature recalled the underwater search vehicle after it exceeded its operating depth of 4,500 meters (14,763 feet).

Capt. Mark Matthews, who heads the U.S. presence in the search effort, said the Bluefin aborted so the crew could refine the mission parameters. Charts indicated the ocean was at most 4,400 meters (14,436 feet) deep, so when the Bluefin went deeper than that, it was returned to the surface.

"It happened in the very far corner of the area it's searching, so they are just shifting the search box a little bit away from that deep water and proceeding with the search," he told CNN's "Erin Burnett OutFront."

The decision to put the Bluefin-21 into the water for the first time in the 38-day search comes nearly a week after listening devices last heard sounds that could be from locator beacons attached to the plane's "black boxes."

"We haven't had a single detection in six days," Australian chief search coordinator Angus Houston said. "It's time to go underwater."

The probe is equipped with side-scan sonar -- acoustic technology that creates pictures from the reflections of sound. Such technology is routinely used to find sunken ships and was crucial in finding Air France Flight 447, which crashed into the Atlantic Ocean in 2009.

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

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Houston cautioned against hopes that the underwater vehicle will find wreckage of the plane, which disappeared on March 8 on a flight between Kuala Lumpur, Malaysia, and Beijing that should have taken about six hours.

"It may not," he said. "This will be a slow and painstaking process."

The bottom of the search area is not sharply mountainous -- it's more flat and almost rolling, Houston said. But he said the area probably has a lot of silt, which can "complicate" the search.

The search area for the Bluefin is in what officials labeled the most likely area the plane could be.

"It was the site of the highest signal strength or the loudest received signal," Matthews told CNN, referring to the pings detected by a towed pinger locator, a wing-shaped listening device connected to the ship by a cable. "It was also where we had an increase in signal strength as we drove through the area and then a decrease as we drove out of the area."

New clue on the surface of the water?

Another possible clue into the plane's disappearance emerged Monday.

Australian officials announced the Australian ship Ocean Shield had detected an oil slick Sunday evening. It is unclear where the oil came from. A 2-liter sample has been collected for examination, but it will take a few days to analyze.

"I stress the source of the oil has yet to be determined, but the oil slick is approximately 5,500 meters (3.4 miles) downwind ... from the vicinity of the detections of the TPL on Ocean Shield," Houston said.

CNN Aviation Analyst Les Abend, who flies a Boeing 777, said the engines on the plane have about 20 quarts of oil each.

"It could be slowly dripping up to the surface," he told CNN's "Anderson Cooper 360." "They're saying an oil slick. I'm wondering if it's just some sort of a fluid slick. It could be (from) hydraulics."

If it is oil, it's not the first oil slick detected as part of the search. A similar find in the first days of the search was determined to be fuel oil from a freighter.

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Surface search nearing end

Twelve aircraft and 15 ships participated in Monday's search efforts on the surface, covering an 18,400-square-mile (47,600-square-kilometer) area. The surface search was among the last, Houston said.

"The air and surface search for floating material will be completed in the next two to three days in the area where the aircraft most likely entered the water," Houston said.

That search was energized last week when searchers using the Navy-owned pinger locator and sonobuoys detected sounds that could have been from the plane's black boxes, or data and voice recorders.

But after a week of silence, the batteries powering the locator beacons are probably dead, a top official from the company that manufactures the beacons told CNN on Sunday. They were certified to last 30 days, a deadline that's already passed.

That means searchers may not be able to detect any more pings to help lead them to those pieces of the missing plane.

"More than likely they are reaching end of life or already have. If (a beacon) is still going, it is very, very quiet at this point," Jeff Densmore told CNN's "State of the Union with Candy Crowley" on Sunday.

The time is ripe to move on to other search techniques.

"Every good effort has been expended, but it's now looking like the batteries are failing, and it's time to start mowing the lawn, as we say, time to start scanning the sea floor," said Rob McCollum, a CNN analyst and ocean search specialist.

Catherine Tamoh Lion, the mother of the missing plane's chief steward Andrew Nari, said the news that no more pings have been heard is upsetting.

"Our sadness is now just prolonged," she told CNN.

"I feel like they are somewhere," she said of the passengers. "I don't know where. Just praying to God. Miracles can happen. "