When aircraft wreckage like the battered remains of a Russian passenger jet that went down Saturday in Egypt is spread over a wide area on the ground, there’s little doubt that the plane broke up in the sky.

What is far more difficult to explain is how an airliner built to withstand the most extreme turbulence possible and equipped with computerized flight limits to ensure it never loses control could have been ripped to pieces.

“It’s very hard to pull one of these things apart in flight. Very hard,” John Cox, a former U.S. airline pilot who has participated in accident investigations, said in an interview.

The Metrojet Airbus Group SE A321 plummeted into a remote area of Egypt’s Sinai peninsula 23 minutes after leaving the Red Sea resort of Sharm el-Sheikh on a flight to St. Petersburg, killing all 224 people aboard. Wreckage was found in an area about 8 kilometers long and 4 kilometers wide (5 miles long by 2.5 miles wide), suggesting the aircraft broke up at a high altitude, Alexander Neradko, the head of the Russian Federal Aviation Authority, said in an interview with Rossiya-24 state television.

If that’s true, according to Cox and other safety consultants, investigators will be looking at a handful of things that have caused jetliners to come apart like that in recent decades: bombs, missiles, other on-board explosions and structural failures.

Not Credible

While the Islamic State’s Sinai affiliate claimed responsibility for shooting the plane down, Egyptian and Russian officials said those claims weren’t credible. All but the most sophisticated ground-based missiles can’t reach 31,000 feet (9,450 meters), the cruising altitude at which the Metrojet encountered problems and began to fall.

That doesn’t rule out a bomb like the one that detonated aboard Pan Am Flight 103 as it was carrying holiday travelers from London to New York on Dec. 21, 1988. A small explosive device smuggled aboard in checked luggage blew out the side of the Boeing Co. 747 and it came apart over Scotland, according to the U.K.’s Air Accidents Investigation Branch report.

So far, neither Egyptian nor Russian officials have said there’s any evidence of a bomb. Explosive devices cause telltale pitting on nearby metal and also leave chemical residue, according to the U.S. National Transportation Safety Board, so an examination of the wreckage should tell investigators whether or not that was the cause.

One area investigators will pay close attention is damage to the Metrojet A321 when its tail struck the runway while landing in Cairo in 2001. The plane was repaired and returned to service, according to Ascend Worldwide Ltd., a London-based company that gathers data for insurers.

There have been at least two similar accidents caused by improper repairs after tail damage.

Tail Strikes

China Airlines Flight 611, a Boeing 747 flying from Taiwan to Hong Kong on May 25, 2002, broke apart when a repair failed, causing an explosive decompression, according to Taiwan’s Aviation Safety Counsel. All 225 people aboard died when it fell into the Taiwan Strait. The jet’s tail had been repaired 22 years earlier, according to the investigation.

Japan Airlines Flight 123 crashed into a mountain in Japan on Aug. 12, 1985, after a repair to its tail came apart, destroying key flight control surfaces. Seven years earlier, its tail had been repaired after striking the ground during touchdown, according to Japan’s Aircraft Accident Investigation Commission. The crash killed 520 out of 524 people on board.

In both cases, part of the aircraft structure that holds in air at high altitudes, known as the aft pressure bulkhead, was damaged when the planes’ tails scraped the runway. Some photos of the wreckage in Egypt appear to show that the plane’s tail section fell separately from the rest of the plane.

If the A321 being operated by Metrojet lost its aft pressure bulkhead, “it would cause very, very serious controllability problems,” Cox said.

Another cause of midair breakups has been explosions in aircraft fuel tanks, according to Steve Wallace, former chief of the U.S. Federal Aviation Administration’s accident investigation division. TWA Flight 800, another 747, crashed into the Atlantic Ocean near New York on July 17, 1996, as a result of such an explosion, killing all 230 aboard, according to the NTSB.

Black Boxes

Teams examining the wreckage in Egypt have found the two so-called black box recorders that capture pilots’ comments in the cockpit and store a detailed record of how the plane functioned, according to a statement by that country’s Civil Aviation Ministry. That will provide important clues about what happened, said Paul Hayes, director of safety at Ascend Worldwide.

“They’ve got the wreckage, they’ve got the recorders, they’ve got the air-traffic-control recordings, so hopefully they should start being able to put together some kind of sequences of events or circumstances leading to the impact,” Hayes said.

As is often the case after a major air disaster, accounts of what happened were at times sketchy and confusing. There were contradictory reports about whether the crew made a distress radio call and officials have not detailed where individual parts of the plane landed.

The initial crash data should be viewed skeptically, Robert Mann, a former American Airlines executive who is president of R.W. Mann & Co. and an aviation consultant, said in an interview.

“Unless there’s something that says ‘obvious,’ you generally have to wait until you get the reports from the recorders,” Mann said.