The Russian jetliner that crashed shortly after takeoff from an Egyptian resort city broke up at high altitude, scattering fragments of wreckage over a wide area in the Sinai Peninsula, Russia's top aviation official said Sunday as search teams raced to recover the bodies of the 224 people who died.

Meanwhile in Russia, an outpouring of grief gripped the historic city of St. Petersburg, home of many of the victims. President Vladimir Putin declared a nationwide day of mourning, and flags flew at half-staff.

Aviation experts joined the searchers in a remote part of the Sinai, seeking any clues to what caused the Metrojet Airbus A321-200 to plummet abruptly from about 9,400 metres just 23 minutes after it departed from the Red Sea resort of Sharm el-Sheikh bound for St. Petersburg.

Aviation experts and the search teams were combing an area of 16 square kilometres to find bodies and pieces of the jet.

Russian airliner's crash site2:58

By midday, 163 bodies had been recovered, according to the Egyptian government. Some of the dead were expected to be flown to Russia later Sunday.

In St. Petersburg, hundreds of mourners brought flowers, pictures of the victims, stuffed animals and paper planes to the city's airport. Others went to churches and lit candles in memory of the dead.

Elena Vikhareva had no relatives aboard the flight, but she went with her son to lay flowers, saying that pain was "piercing" her heart.

Vladimir Povarov and a friend did the same, explaining that they couldn't "remain indifferent."

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An Egyptian army soldier stands guard near debris from a Russian airliner at its crash site. Russia has grounded Airbus A321 jets flown by the Kogalymavia airline, after one of its fleet crashed in Egypt's Sinai Peninsula. (Mohamed Abd El Ghany/Reuters)

The large area over which fragments were found indicates that the jet disintegrated while flying high, said Alexander Neradko, head of Russia's federal aviation agency. He would not comment on any possible reason for the crash, citing the ongoing investigation.

Neradko was in Egypt to inspect the crash site along with Russia's ministers of emergencies and transport.

Transport Minister Maxim Sokolov thanked Egyptian authorities for their help and said work on analyzing the data and cockpit voice recorders had not yet begun.

'It was a good plane'

An Egyptian ground service official who carried out a preflight inspection of the plane said the aircraft appeared to be in good shape. Speaking on condition of anonymity because he was not authorized to speak to the media, he said he was a member of a technical inspection team that included two Russians.

"We are all shocked. It was a good plane. Everything checked out in 35 minutes," the official told The Associated Press on Sunday. The closest the plane came to being in trouble, he said, was three months ago when the pilot aborted takeoff halfway through because of a system error. "That's almost routine though," he said.

However, a Russian TV channel late Saturday quoted the wife of the co-pilot as saying her husband had complained about the plane's condition. Natalya Trukhacheva, identified as the wife of Sergei Trukhachev, said a daughter "called him up before he flew out. He complained before the flight that the technical condition of the aircraft left much to be desired."

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Egyptian Army soldiers stand near passenger luggage a day after a jet bound for St. Petersburg, Russia, crashed in Egypt. The Metrojet plane crashed 23 minutes after it took off from Egypt's Red Sea resort of Sharm el-Sheikh. (Amr Nabil/Associated Press)

An Egyptian official had previously said that before the plane lost contact with air traffic controllers, the pilot radioed that the aircraft was experiencing technical problems and that he intended to try to land at the nearest airport.

Most airline accidents occur in the early or late stages of a flight, around takeoff or landing, according to Todd Curtis, a former safety engineer with Boeing.

"Once you're in cruise, many of the risks that exist at lower altitudes, from bird strikes to running into things, don't happen," said Curtis, who is now director of the Airsafe.com Foundation. "And ultimately, when you have a problem in cruise, you typically have time to fix it."

Weather, collision or explosion?

When planes do break up in midair it's usually because of one of three factors: a catastrophic weather event, a midair collision or an external threat, such as a bomb or a missile.

With no indication that those events played a role in the crash, Curtis said investigators will be looking at more unusual events, such as an on-board fire or corrosion that caused a structural failure.

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A man helps a young girl lay flowers at an entrance of Pulkovo airport outside St.Petersburg, Russia, during a day of national mourning for the 224 crash victims, on Sunday, Nov. 1, 2015. (Dmitry Lovetsky/Associated Press)

The flight recorders will provide key information, including the plane's airspeed and whether it was on autopilot.

"The good news is with the recorders in hand, both the French and Russian investigators should have a good idea in very short time what did occur," said Jim Hall, former chairman of the U.S. National Transportation Safety Board.

Alexander Smirnov, Metrojet's deputy director, said an engine failure would not have caused the plane to crash.

"An engine failure doesn't lead to catastrophe," he said on television. Smirnov described the A321 as a reliable aircraft that would not fall into a spin even if the pilots made a grave error because automatic systems correct crew mistakes.

Egyptian President Abdel-Fattah el-Sissi cautioned that the cause of the crash may not be known for months.