Many of the passengers on the Airbus A321-200 aircraft, which crashed en route from the Egyptian resort of Sharm el-Sheikh to St. Petersburg, were reported by Russian state media to be returning from vacation. Russian officials said there were 25 children aboard the plane.

At Pulkovo Airport in St. Petersburg, where the aircraft was supposed to end its journey, mourners paid their respects to victims at a makeshift memorial. People brought red or white carnations and stuffed toys. A table held a dozen candles. Relatives who had waited desperately for news of loved ones broke down in tears.

Russian President Vladimir Putin has declared Sunday a day of mourning.

It remains unclear what caused Flight 9268 to suddenly drop off radar, in clear weather after only 23 minutes in the air, and hurtle to the ground.

Speaking to high-ranking army officers in Cairo on Sunday, Egyptian President Abdel Fattah el-Sisi asked them to observe a moment of silence before urging the public not to jump to conclusions. Determining what happened will require a lengthy investigation, he said.

"These are complicated matters that require advanced technologies and wide investigations that might go on for months," he said.

The crash is most likely the result of a technical failure, Egyptian Airports Co. chief Adel Al-Mahjoob told CNN Arabic on Saturday, although he noted that the plane passed a routine check before it took off.

Russian media outlets said that the pilot reported technical problems and requested a landing at the nearest airport before the plane went missing, but Egyptian authorities disputed that claim.

Air traffic control recordings don't show any distress calls, Egyptian Civil Aviation Minister Hossam Kamel said at a news conference.

"There was nothing abnormal before the plane crash," he said. "It suddenly disappeared from the radar."

CNN aviation correspondent Richard Quest said it was "unusual" for an aircraft to go down after around 20 minutes in the sky.

"At this point, a plane is on autopilot. It's reaching its initial cruising altitude, and there is little that can or should go wrong," he wrote in an analysis.

Investigators are likely to get a better understanding of what happened from the aircraft's so-called black boxes -- the flight data recorder and cockpit voice recorder -- which have been recovered and transported to Cairo for analysis.

The data recorder stores a vast array of information about the flight, such as air speed, altitude, engine performance and wing positions. The voice recorder captures sounds on the flight deck that can include conversations between the pilots and warning noises from the aircraft.

Analysis: Plane crashed at what should have been flight's safest point

The Sinai Peninsula, where Flight 9268 crashed, is home to ISIS-affiliated militants who are locked in a deadly conflict with Egyptian security forces. They appeared to claim responsibility for bringing down the Russian passenger jet in a statement posted online Saturday, but officials in Egypt and Russia dismissed it.

Mahjoob, the airport official, said there was no evidence of a terrorist attack. And Russian Transport Minister Maxim Sokolov said the claim that terrorists brought down the plane by using an anti-aircraft missile "cannot be considered reliable," according to the Russian state news agency RIA Novosti.

The Egyptian military said militants in Sinai have shoulder-fired anti-aircraft weapons that only shoot as high as 14,000 feet, far short of the more than 30,000 feet at which Flight 9268 was flying when it dropped off radar.

To reach such an altitude would require missiles using special launch pads and radar systems operated by engineers, the military said.

It added that many of the victims of the crash were found with their seatbelts on, suggesting the pilot had asked them to buckle up because of a problem with the aircraft.

Nonetheless, Air France, the German air carrier Lufthansa and the UAE airlines Etihad, Emirates, AirArabia and flydubai have decided to reroute aircraft scheduled to fly over Sinai.

"We will keep that measure in place as long as we are not sure of the circumstances and the reasons of the Metrojet crash," Lufthansa spokeswoman Bettina Rittberger said. Metrojet is the name by which the Russian airline, Kogalymavia, is commonly known.

Etihad said in a statement that it was "complying with instructions by the Egyptian authorities to avoid certain areas of airspace over the Sinai peninsula," which would affect only a handful of flights.

Russian emergency ministry officials were on the ground at the crash site in northern Sinai on Sunday, Russian state media reported.

Sisi, the Egyptian President, has promised Putin to allow "the broadest possible participation of Russian experts in the investigation," according to the Kremlin. Putin has also ordered Russian Prime Minister Dmitry Medvedev to open an investigation into the crash, it said.

The Egyptian government said Sunday that 163 bodies have so far been transported to morgues and hospitals in Cairo while the search continues for others at the site. Russian government aircraft were expected to start transporting them to St. Petersburg.

The Airbus A321-200 that crashed Saturday was built in 1997 and Metrojet had been operating it since 2012, Airbus said in a statement. The aircraft had clocked up around 56,000 flight hours over the course of nearly 21,000 flights, it said.

There were 217 passengers and seven crew members on board, most of them believed to be Russian. Ukrainian Foreign Minister Pavlo Klimkin tweeted that four victims were Ukrainian citizens.

Sharm el-Sheikh, where Flight 9268 began its journey, is a beach resort dotted with palm trees at the southern tip of the Sinai Peninsula. The plane crashed about 300 kilometers (185 miles) farther north, near a town called Housna, according to Egyptian authorities.