“We don't have any direct evidence of any terrorist involvement yet” in the crash, Clapper told reporters in Washington on Monday. Saturday that killed all 224 people on the Metrojet.
But he noted that ISIL group, which has claimed responsibility, has a significant presence in Egypt's Sinai Peninsula. Asked if ISIL had the capabilities to bring down a passenger jet, he said, “It's unlikely, but I wouldn't rule it out.”
Robert Galan, a French aviation expert, said comments by an executive at the Russian airline Metrojet that an “external impact” brought down its plane in the Sinai point to two possibilities: a bomb or sabotage.
Galan, who has written a history of airline disasters, says he was not familiar with security at the Sharm el-Sheikh airport where the plane took off Saturday before crashing 23 minutes later. But Galan told The Associated Press that any plane on a tarmac can be surrounded by more than a dozen mechanics or other technicians.
Sabotage would require familiarity with the electrical or fuel systems of the A320-200, but he said hiding a bomb would need less knowledge.
Galan said analysis of the black box will not confirm either a bomb or sabotage, as it records only the pilots' communications and technical readings. But he said investigators could know within 48 hours whether a bomb downed the jet, because the debris would show traces of explosives.
Alexander Smirnov, deputy general director of Metrojet said Monday, “We rule out a technical fault of the plane or a pilot error. The only possible explanation could be an external impact on the airplane.”
When pressed for more details about the type of impact and what could have caused it, Smirnov insisted that he was not at liberty to discuss details because the investigation was ongoing. He also did not explain whether he meant something had hit the plane or that some external factor caused the crash.
Viktor Yung, another deputy director general of Metrojet, said the crew did not send a distress call and they did not contact traffic controllers before the crash.
The Airbus A321-200 crashed from 31,000 feet in the Sinai Peninsula just 23 minutes after taking off from the Egyptian Red Sea resort of Sharm el-Sheikh en route to St. Petersburg. Smirnov said the plane dropped 186 mph in speed and about 5,000 feet in altitude one minute before it crashed.
Metrojet's deputy general director for engineering, Andrei Averyanov, said a 2001 incident when the plane's tail section struck the tarmac on landing was fully repaired and could not have been a factor in the crash.
He said the aircraft's engines had undergone routine inspection in Moscow on Oct. 26 which found no problems and he said in the five flights before the crash, the crew recorded no technical issues in the aircraft's log book.
On Sunday, Neradko told reporters that the large area over which plane debris fragments were found indicated that the jet disintegrated while flying at high altitude. He also would not comment on any possible reason for the crash, citing the ongoing investigation.
When planes do break up in midair, experts say 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, Todd Curtis, a former safety engineer with Boeing, said investigators would be looking at more unusual events, such as an on-board fire or corrosion that caused a structural failure.
British military analyst Paul Beaver said he thought the crash was most likely caused by a bomb on board, saying he was certain ISIL does not possess a missile system — such as the Russian Buk — capable of hitting the plane.
That's a very serious piece of equipment, and I don't think they have that sophistication,” he said.
He also said the Sinai desert is well-scrutinized by intelligence agencies, so a missile would have been seen.
Similarly, Russian officials have discounted ISIL's claim of responsibility as not credible.
The flight recorders will provide key information, including the plane's airspeed and whether it was on autopilot. Russian officials were shown the black boxes found at the site of the crash in the Sinai Peninsula and emergency situations minister Vladimir Puchkov says they are in a good condition, Russian news agencies reported on Monday.
At the crash site, emergency workers and aviation experts from Russia and Egypt swept across the barren terrain Monday, searching for more victims and examining the debris for more clues as to the cause of the crash.
A Russian cargo plane brought the first bodies of Russian victims killed in the crash to St. Petersburg, where many of them are from. The city is holding three days of mourning through Tuesday.
The government plane brought 140 bodies to St. Petersburg's Pulkovo airport, touching down in the dark. The bodies were then taken to a city morgue and a crematorium, where Russian forensic experts immediately began working to identify the victims, said Yulia Shoigu, a Russian Emergency Situations official.
The search for bodies at the Sinai crash site should wrap up late Monday night and another plane with more crash victims' bodies will then travel from Cairo to St. Petersburg, Puchkov told a televised news conference.
President Vladimir Putin declared Sunday a nationwide day of mourning and flags flew at half-staff across the country.
Mourners have been coming to St. Petersburg's airport since Saturday with flowers, pictures of the victims, stuffed animals and paper planes. Others went to churches and lit candles in memory of the dead.
In the Sinai, aviation experts and search teams have been combing an area of more than six square miles to find bodies and pieces of the jet. The Egyptian government said Sunday that 163 bodies had been recovered.
Russia has sent over 100 emergency workers to Egypt to help with the investigation into the crash, and aviation teams from France, Germany and Airbus are also working in Egypt.
Prime Minister Dmitry Medvedev on Monday urged international experts to get involved in the investigation.
Smirnov, Metrojet's deputy director, described the A321 as a reliable aircraft that would not fall into a spin even if the pilots made a grave error because its automatic systems would correct crew mistakes.
Egyptian President Abdel-Fattah el-Sissi cautioned that the cause of the crash might not be known for months.
“It's very important that this issue is left alone and its causes are not speculated on,” he told a meeting of top government officials. The investigation “will take a long time" and “needs very advanced technologies.”