Story highlights Search agency: The fifth signal is unlikely to be from the black boxes

"There has been no major breakthrough," the head of the agency says

Friday's search is focused on a smaller area

A U.S. Navy supply ship is joining the search for the missing plane

Elevated hopes that Malaysia Airlines Flight 370 might soon be found were tempered Friday, when the joint search agency said the latest signal probably isn't from the missing plane.

The most recent acoustic signal detected by an Australian aircraft in the search Thursday is "unlikely to be related to the aircraft black boxes," Australian chief search coordinator Angus Houston said in a statement Friday.

"On the information I have available to me, there has been no major breakthrough in the search for MH370," Houston said.

"Further analysis continues to be undertaken by Australian Joint Acoustic Analysis Centre."

But Australian Prime Minister Tony Abbott told reporters in China on Friday that authorities are "very confident" the signals picked up by acoustic detectors are coming from the black box of missing Malaysia Airlines Flight 370, CNN affiliate Sky News Australia reported.

It's unclear whether Abbott was referring to four signals detected earlier this week.

As planes and boats scoured the Indian Ocean for more signals and signs of wreckage, a senior Malaysian government official and another source involved in the investigation divulged details about the flight to CNN on Thursday, including new information about what radar detected, the last words from the cockpit and how high the plane was flying after it went off the grid.

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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

Malaysia Airlines Flight 370 disappeared from military radar for about 120 nautical miles after it crossed back over the Malay Peninsula, sources say. Based on available data, this means the plane must have dipped in altitude to between 4,000 and 5,000 feet, a senior Malaysian government official and a source involved in the investigation tell CNN.

The dip could have been programmed into the computers controlling the plane as an emergency maneuver, said aviation expert David Soucie.

"The real issue here is it looks like -- more and more -- somebody in the cockpit was directing this plane and directing it away from land," said CNN aviation analyst and former National Transportation Safety Board Managing Director Peter Goelz. "And it looks as though they were doing it to avoid any kind of detection."

But former U.S. Department of Transportation Inspector General Mary Schiavo was not convinced. She said the reported dip could have occurred in response to a loss of pressure, to reach a level where pressurization was not needed and those aboard the plane would have been able to breathe without oxygen, or to get out of the way of commercial traffic, which typically flies at higher altitudes.

That would have been necessary had the plane's transponder been turned off and it lost communications. "If you don't have any communications, you need to get out of other traffic," Schiavo said.

"We still don't have any motive and any evidence of a crime yet," she said, adding that most radar can track planes at altitudes below 4,000 feet, so the plane's descent may not have indicated any attempt by whoever was controlling it to hide.

She held out hope that the black boxes hold the answers and that they will be found soon.

New flight details revealed

Malaysian sources told CNN that Flight 370's pilot, Capt. Zaharie Ahmad Shah, was the last person on the jet to speak to air-traffic controllers, telling them "Good night, Malaysian three-seven-zero."

The sources said there was nothing unusual about his voice, which betrayed no indication that he was under stress.

One of the sources, an official involved in the investigation, told CNN that police played the recording to five other Malaysia Airlines pilots who knew the pilot and copilot.

"There were no third-party voices," the source said.

The sources also told CNN that Malaysian air force search aircraft were scrambled about 8 a.m. March 8 to the South China Sea and the Strait of Malacca, soon after Malaysia Airlines reported that its plane was missing. The aircraft took off before authorities corroborated data indicating that the plane turned back westward, a senior Malaysian government official told CNN.

But the air force did not inform the Department of Civil Aviation or search and rescue operations until three days later, March 11, a source involved in the investigation told CNN.

Possible signal raises hope

The possible signal heard by a search plane was picked up through sonar buoys equipped to receive such electronic data and was detected near the Australian ship Ocean Shield, said the Joint Agency Coordination Centre

The Australian Defense Force source said the signal detected was not at the 37.5 kHz frequency consistent with the pingers from flight data recorders but in a range that suggests strongly that it is from something that is man-made. Commodore Peter Leavy of the Royal Australian Navy said Wednesday in Perth that existing technology in RAAF P3 aircraft had been modified to allow the acoustic processor to pick up sounds in the frequency range. Using the technology in this way is experimental, according to the source.

The source said four RAAF P3 Orions have been modified with this technology, with the sonar buoys expiring and sinking about eight hours after they are deployed from the aircraft. On Wednesday, Leavy said that each P3 is capable of deploying 84 buoys, laid in a pattern or grid coordinated with the Ocean Shield.

Although Leavy said the buoys have sensors that can detect signals "at least" 1,000 feet below the surface, the source is confident that the technology has been tested at a "much deeper depth."

Crews have been narrowing the search area in the Indian Ocean.

Search areas shrinks

Up to 12 military aircraft, three civil aircraft and 13 ships were assigned to assist in Friday's search for the Boeing 777-200ER, which was carrying 239 people when it vanished March 8 on a fight from Kuala Lumpur, Malaysia, to Beijing.

There were no sightings reported by search aircraft or objects recovered by ships on Thursday, the Joint Agency Coordination Centre said.

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Friday's search area was about 18,000 square miles (46,700 square kilometers), centered 1,436 miles (2,312 kilometers) northwest of Perth.

That's far smaller than the search area's size a few weeks ago.

"It's pretty incredible, if you look at where we started, which was virtually the entire Indian Ocean, now getting it down to what's essentially a couple hundred square miles (where the pings have been detected) is pretty miraculous," Marks said.

The Ocean Shield first picked up two sets of underwater pulses on Saturday that were of a frequency close to that used by the locator beacons. It heard nothing more until Tuesday, when it reacquired the signals twice. The four signals were within 17 miles of one another.

"I believe we are searching in the right area, but we need to visually identify wreckage before we can confirm with certainty that this is the final resting place of MH370," Houston said Wednesday.

As the search continues, a U.S. Navy supply ship will help provide supplies and fuel to the ships that are looking for the missing plane.

The USNS Cesar Chavez will help supply Australian naval ships involved in the search "in the coming days," the Navy said in a statement.

That's likely a sign that search teams are preparing for a lengthy hunt, analysts said.

"I think they're getting ready for the long haul," Goelz said. "Even if they do get four or five more pings, once they drop the side-scanning sonar device down, that is going to be painstaking and long. So I think they are settling in for the long search."

Friday is Day 35 in the search. Time is of the essence: The batteries powering the flight recorders' locator beacons are certified to emit high-pitched signals for only 30 days after they get wet.

"The signals are getting weaker," Houston said Wednesday, "which means we're either moving away from the search area or the pinger batteries are dying."