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Sharm el-Sheikh, Egypt • 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 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 31,000 feet 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 more than 6 square miles to find bodies and pieces of the jetliner.

By midday, 163 bodies had been recovered, according to the Egyptian government.

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.

The large area over which fragments were found indicates the jetliner 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 emergency and transport ministers.

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, 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.

But 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."

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, 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."

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.

A local affiliate of the extremist Islamic State claimed it brought down the aircraft, which crashed in the same general area of northern Sinai where the Egyptian military and security forces have battled militants for years. The claim, in a statement posted on social media, provided no supporting evidence. The militants are not known to be capable of downing an aircraft at cruising altitude, although there have been media reports that they have acquired Russian shoulder-fired, anti-aircraft missiles that are effective against low-flying warplanes or helicopters. Lufthansa, Air France, Dubai-based Emirates and Qatar Airways said they would suspend flights over the Sinai until the reason for the crash is known. British Airways, EasyJet and Virgin Atlantic said they were operating as usual in the region.