Story highlights Official says Bluefin may be able to go deeper; another vehicle that can awaits its call

Underwater probe will map ocean floor until 10 a.m. ET Wednesday, source says

Vehicle's first attempt ended early after it exceeded its maximum dive depth

Meanwhile, investigators are trying to determine significance of cell signal and oil slick

The underwater probe being used to look for missing Malaysia Airlines Flight 370 was back in the water after its first attempt ended prematurely, said the company that owns the vehicle, Phoenix International.

The Bluefin-21 autonomous underwater vehicle was about four hours into its second dive mission at 2 p.m. ET on Tuesday (2 a.m. Wednesday in Perth, Australia), a source close to the operation told CNN's Brian Todd.

On Monday, crews sent the probe toward the ocean floor on what was expected to be a 20-hour deployment, only to have it return in less than eight hours after encountering waters beyond its 4,500-meter (14,764-foot) maximum depth.

The probe found no debris during its shortened scanning session.

The second mission is expected to end Wednesday around 10 a.m. ET (10 p.m. in Perth), the source said. The vehicle was deployed in nearly the same area and is operating at about the same depth as the earlier mission, the source said.

The earlier aborted mission doesn't mean anything is wrong with the probe, which is designed to swim about 30 meters (100 feet) above the ocean floor and use sound waves to draw a three-dimensional map of what lies below.

"The vehicle's tracking the floor, so when the floor dives, so does the vehicle. And the vehicle goes, 'Uh oh, I'm not supposed to be here' and punches up," said David Gallo, director of special projects at the Woods Hole Oceanographic Institution.

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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

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And while it's disappointing the vehicle returned to the surface early, it's not unusual, said David Kelly, CEO of Bluefin Robotics , the company that makes the probe.

"We've operated these vehicles around the globe. It's not unusual to get into areas where the charts aren't accurate or you lack information," Kelly told CNN's "Erin Burnett OutFront."

A British survey ship is working to determine the areas where the Bluefin can safely work without hitting its maximum dive depth, according to Phoenix.

U.S. Navy Capt. Mark Mathews of the Bluefin search team said the initial launch Monday night took place "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."

About 8 square miles were searched, said Jim Gibson, a spokesman for Phoenix International. It could take up to two months to scan the entire search area.

The deputy director for salvage and diving for the U.S. Navy told CNN that the Bluefin may actually be able to go deeper than it was first programmed to do, engineers have determined.

"We've gone through and looked at all of the components in Bluefin, and we're comfortable that we can exceed that 4,500-meter limit at this point," Mike Dean told CNN's "The Lead with Jake Tapper. "We're looking at, can we push Bluefin beyond 4,500, potentially down to as deep as 5,000? There is some software, that software has been tested now. And we believe that, with some confidence, we could push Bluefin to that depth."

He said two-thirds of the search area was less than 4,500 meters deep.

Dean said the Bluefin was chosen for this part of the search because it was the lightest, most nimble package and easiest to get on scene. It's possible that a second vehicle that can operate deeper than the Bluefin will be needed.

One possibility is a vehicle called Orion that is towed behind a ship. It remains under the water and sends back real-time data. It would be a more complex operation to put an Orion in the water but one is available in Maryland if the Australian-led search team requests it.

Monday's shortened mission was the latest glitch in a 40-day search for the missing jetliner, which vanished March 8 with 239 passengers and crew on board after taking off from Kuala Lumpur, Malaysia, bound for Beijing.

Surface and satellite searches have turned up nothing conclusive -- and confusing, sometimes conflicting details from investigators have muddied the public image of the search and angered relatives of the missing.

On Tuesday, the Malaysian Cabinet agreed to set up an international investigation to look into the plane's disappearance. Teams will look at the plane's airworthiness, operational issues and human factors that may have played a role, officials said.

Malaysia's acting Transportation Minister Hishammuddin Hussein also said Tuesday that if searchers are able to recover the plane's vital black boxes, it matters less which country takes control of them than does "finding out the truth."

The co-pilot's cell phone

Meanwhile, a new detail emerged from the flight on Monday.

A U.S. official told CNN that co-pilot Fariq Abdul Hamid's cell phone was on and made contact with a cell tower in Malaysia about the time the plane disappeared from radar.

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

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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.

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.

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.

The suspected oil slick

Another possible clue into the plane's disappearance appeared 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, and was on its way Tuesday to western Australia for analysis. Test results could be days away.

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.

Surface search nearing end

While air and sea surface searches continued Wednesday some 2,087 kilometers (1,297 miles) northwest of Perth, those searches are likely nearing an end.

"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," retired Australian Air Chief Marshal Angus Houston, head of the country's Joint Agency Coordination Centre, said Monday.

With no debris found after weeks of searches and no possible pings from the plane's black boxes detected in a week, Houston said it was time to focus the search underwater.

The number of aircraft in the search grew to 14 planes, including three civilian jets. There were also 11 ships involved Wednesday.