Russian plane crash mystery: The three leading theories

Of particular interest to investigators is the Russian airliner's repair history, according to aviation experts. The 18-year-old plane struck the runway during a landing in 2001, causing 'serious damage' to the tail.

A Russian plane that crashed in Egypt, killing all 224 people aboard, was possibly brought down by "external impact" to the aircraft, an airline official said Monday.

"We rule out a technical fault of the plane or a pilot error," said Alexander Smirnov, deputy general director of Metrojet, the Associated Press reports. "The only possible explanation could be an external impact on the airplane."

Theory 1: A bomb on board? 

"It could have been a bomb," Paul Rogers, a global security consultant and professor at the University of Bradford in England, told the BBC. "The fact is that Russia recently intervened in Syria and … this could be a nasty blowback as far as [President Vladimir] Putin is concerned. The reality is that neither Egypt nor Russia will want to admit it involved terrorism and it may never come out fully."

A Kremlin spokesman told the BBC that terrorism has not been ruled out as a potential cause.

Mr. Smirnov declined to elaborate, saying he would not talk about details of the crash while the investigation is ongoing.

This claim comes a day after Alexander Neradko, the head of Russia's federal aviation authority, told reporters that pieces recovered from the crash site provide evidence that the plane came apart at a high altitude. Another deputy director general of Metrojet, Viktor Yung, said the crew did not issue a distress call or make contact with air traffic control before the crash. 

"We exclude technical problems and reject human error," Mr. Smirnov, said at a Moscow news conference as he discussed possible causes of the crash.

Smirnov said that the plane crashed 23 minutes after taking off from Sharm el-Sheikh, a resort by the Egyptian Red Sea, on its way to St. Petersburg. One minute before crashing, the aircraft climbed briefly, then slowed by 186 mph and dropped about 5,000 feet, according to data from the Flight Radar 24 tracking site.

Smirnov described the aircraft, an Airbus A321, as reliable, and said that pilot error would not cause a catastrophic crash because fail-proof technology would correct crew mistakes, the BBC reports.

Theory 2: A faulty tail section?

Aviation experts say that investigators are probably scrutinizing the plane's repair history. In 2001, the plane struck the runway during a landing in Cairo, resulting in a "hard tail-strike on landing, causing serious damage", according to aviation publication Flight Global.

Two other planes have broken up in midair as a result from prior damage, including the 1985 Japan Airlines crash, the second-deadliest in history, which experienced a catastrophic decompression 12 minutes into the flight, killing 505 of 509 on board, seven years after damage to the tail was repaired. Faulty repairs were determined to be the cause of that crash.

The aft pressure bulkhead serves an important purpose in flight: the airtight component separates an airliner's passenger cabin and the tail section of the plane, providing a seal, and therefore keeps the cabin pressure consistent during flight.

Smirnov insists the Russian airliner was fit for flight.

Theory 3: ISIS shot it down with a missile?

A local Islamic State group quickly took responsibility for bringing the aircraft down. But that claim that has been rejected by Russian officials and military experts as implausible because the group lacks the missiles to shoot down an aircraft at 31,000 feet. Islamic extremists have clashed with Egyptian forces since the Egyptian Revolution in 2011, part of the so-called Arab Spring that brought down a dictatorship in Egypt. 

Egyptian President Abdel-Fattah el-Sissi has tempered such theories, warning that the cause of the crash may remain a mystery 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, according to the AP. He said the investigation "will take a long time" and "needs very advanced technologies."

The black boxes were recovered from the wreckage successfully and are being investigated by aviation experts, Russian news outlets report. Analysis of the data recorders should allow investigators to determine if it was an explosion or a technical failure that caused the plane to break apart.

Some 100 emergency workers from Russia have arrived in Egypt to assist with the investigation of the crash site, which covers about seven square miles, alongside teams from France, Germany, and Airbus. Russian officials says investigators had finished combing the original crash site, and have expanded the scope of the search for victims to an 11.6-mile radius. 

As of Sunday, 163 bodies had been recovered, Egyptian officials said.