Russia's top aviation official said Sunday that the jet which crashed in Egypt, killing all 224 people on board, broke up at a high altitude.
The head of the country's federal aviation agency, Alexander Neradko, said the large area over which fragments of the plane were scattered indicate that it disintegrated while flying high. Neradko, in Egypt to inspect the crash site, wouldn't comment on a possible cause of the crash, saying the probe is ongoing.
Russia has also sent about 100 emergency workers to the Sinai Peninsula to help search for bodies and examine debris from the crash of the passenger jet.
The Metrojet charter flight, whose passengers included 17 children, crashed early Saturday, 23 minutes after taking off from the Sinai Red Sea resort of Sharm el-Sheikh en route to St. Petersburg in Russia. It went down in a desolate mountainous area of central Sinai.
The plane's two black boxes, one for cockpit voice recordings and one for flight data, have been recovered for use in determining the cause of the crash.
A former Boeing safety engineer said that when planes 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 any such factor played a role in the crash, Curtis said investigators will be looking at more unusual possibilities, such as an on-board fire or corrosion that could cause a structural failure.
"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.
Hundreds of people brought flowers and pictures to St. Petersburg's airport to commemorate the 224 victims.
Elena Vikhareva, 48, came Sunday with her son to lay flowers, saying "pain is piercing her heart" even though she had no relatives on the plane. Vladimir Povarov, 19, brought flowers with a friend, saying "we couldn't remain indifferent."
Russia's air safety regulator has ordered the Metrojet airline to ground its fleet of Airbus A321s, the type that crashed. The air safety agency said the carrier must analyze the situation and weigh all risks before it will be allowed to resume flights.
On Saturday, the wife of the plane's co-pilot said her husband had complained about the plane's condition, according to a Russian TV channel.
Russia's state-controlled NTV ran an interview with Natalya Trukhacheva, identified as the wife of co-pilot Sergei Trukachev. She said that 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 ground-service official who inspected the plane said it had appeared to be in good technical condition.
Sinai is the scene of an insurgency by supporters of the Islamic State in Iraq and the Levant (ISIL). Rebels have killed hundreds of Egyptian soldiers and police and have also attacked Western targets in recent months.
Egyptian security sources said there was no indication that the Airbus jet had been shot down or blown up.
Four major airlines — Air France, German carrier Lufthansa, Qatar Airways and the Dubai-based Emirates — have announced that they will avoid flying across the Sinai until there is more information about the cause of the crash.
An Air France spokeswoman said the airline's flights would avoid the area for the time being "as a precaution, until further notice." The airline will instead use other routes to the region's airports.
In addition, Germany's transport ministry has warned all of the country's airlines to avoid flying over both the northern and southeastern parts of the Sinai Peninsula until more details are known about the crash.
Photos from the crash site, 44 miles 70 kilometers south of the city of el-Arish, have shown heaps of smoldering debris dotting the barren terrain, including the plane's badly damaged sky blue tail.
An Egyptian security officer who arrived at the site after the crash called it "a tragic scene. A lot of dead on the ground and many who died whilst strapped to their seats."
"The plane split into two, a small part on the tail end that burned and a larger part that crashed into a rock," the officer, who requested anonymity, said Saturday. "We have extracted at least 100 bodies and the rest are still inside."
The jet, operated by the Russian airline Kogalymavia, which trades as Metrojet, had 217 passengers and seven crew members, all Russian citizens. It took off at 5:51 a.m. Cairo time and disappeared from radar screens 23 minutes later, Egypt's Civil Aviation Ministry said in a statement.
It was at an altitude of 31,000 feet when it vanished from radar screens.
The A321 is a medium-haul jet in service since 1994, with over 1,100 in operation worldwide and a good safety record. It is a highly automated aircraft relying on computers to help pilots stay within safe flying limits.
Saturday's crash is the second fatal accident involving this variant of the A320 jetliner family, according to data from the Flight Safety Foundation.
The Sharm el-Sheikh resort, known for its pristine beaches and scuba diving, is one of Egypt's major attractions and draws millions of tourists, many of them Russian.
It and other resorts dotting the Red Sea coast are heavily secured by the military and police, as conflict rages in north Sinai bordering Israel and the Gaza Strip.
Al Jazeera and wire services