NEW YORK (AP) -- Investigators are still trying to determine what led to the sudden and catastrophic breakup of a Metrojet plane Saturday over Egypt's Sinai Peninsula, just 23 minutes after the Russian-operated jetliner took off from the resort of Sharm el-Sheikh.
The Airbus A321-200 was flying at 31,000 feet when the disaster occurred, killing all 224 people aboard, the vast majority of them Russians.
That is generally the safest part of any flight. Just 9 percent of fatal accidents happen when a plane is at cruising altitude, according to a statistical summary of commercial jet accidents done by Boeing.
Typically, such midair breakups are caused by external threats, such as a bomb or a missile, or a massive structural failure from either metal fatigue or an on-board fire.
Here's a closer look at some of the potential causes of the crash and what investigators will be looking for.
Q: Could this have been a mechanical problem?
A: The airline itself has said no to such a technical failure but investigators have not ruled out anything yet, especially a mechanical issue.
Most aircraft are made of aluminum which is susceptible to corrosion over time. The constant pressurization and depressurization of the cabin for takeoff and landing also puts great stress on the airframe and can eventually lead to tiny cracks. If unchecked, those cracks can grow and threaten the plane's structural integrity.
In April 2011, a Southwest Airlines Boeing 737 made an emergency landing shortly after takeoff from Phoenix, Arizona after the plane's fuselage ruptured, causing a 5-foot tear. The plane, with 118 people on board, landed safely. In 1988, a 19-year-old Aloha Airlines Boeing 737-200 that had made frequent, short hops among the Hawaiian islands lost a large part of its roof. Corrosion and metal fatigue were to blame.
Q: Could a prior repair be to blame?
A: Nothing indicates that faulty work was done following an incident in 2001 in which the tail of the Airbus A321-200 hit the runway. But investigators will be looking into it. If cracks in the tail section were not properly repaired, they could have grown over time and led to a tear under pressure. Jets go through a heavy maintenance check every few years that should have spotted such issues. If the tail were to be lost in flight, pilots would have little control over the jet. In 1985, a Japan Airlines Boeing 747 had an explosive decompression that led to a crash killing 520 people. Investigators eventually blamed the crash on prior faulty repairs.
Q: What about a bomb?
A: Several planes have been brought down, including Pan Am Flight 103 between London and New York in December 1988. There was also an Air India flight in June 1985 between Montreal and London and a plane in September 1989 flown by French airline Union des Transports Aeriens that blew up over the Sahara.
Investigators will look for traces of explosives on the wreckage. They will also use microscopes to look at the tearing patterns on the metals and to see if there are tiny divots, indicative of an explosion.
Q: What else could cause such a breakup?
A: There could have been a massive explosion inside the jet from a spark in the fuel tanks or a rapid fire of something in the cargo hold. The 1996 crash of TWA 800 resulted from an explosion in the center wing fuel tank. That same year, the crash of ValuJet Airlines Flight 592 in the Florida Everglades was blamed on a fire in the cargo hold. More recently, a UPS jet in 2010 crashed after leaving Dubai. Investigators later blamed a fire, traced back to lithium batteries in the cargo hold, for the crash.
Q: Could the plane be shot down by a missile?
A: Probably not. A local affiliate of the extremist Islamic State group has claimed it brought down the aircraft, but intelligence experts say they don't have weapons capable of reaching the 31,000-foot elevation the plane was flying at when the disaster occurred.
Q: What about a midair collision?
A: The crash could have been caused by hitting another jet, but no other planes are reported missing and there does not appear to be debris from another plane, ruling out that scenario.
Q: What can we tell from the wreckage?
A: Parts of the jet are scattered across a 7.7-square mile area. That indicates the plane most likely broke apart at a high elevation. When planes crash intact, the debris field is typically more compact. Also, the parts of the plane tend to be broken into smaller pieces than in this crash.
Q: What is the most important evidence in the wreckage?
A: The black boxes, which are actually orange. The flight-data and cockpit-voice recorders will reveal the plane's speed, altitude, direction and the pilots' actions during the flight. The cockpit recorder will capture the pilots' final words. Investigators will also listen for an explosion and see if any of the critical systems malfunctioned.
Q: Are Russian jets safe?
A: The countries of the former Soviet Union have one of the worst aviation safety records in the world. Only Africa sees more plane crashes than the former Soviet nations, called the Commonwealth of Independent States.
From 2010 to 2014, there were 2.71 jets lost per 1 million flights in those nine nations, compared with a worldwide rate of 0.45, according to the International Air Transport Association. North America has one of the safest records with just 0.13 planes lost per million flights. That means there were 21 times as many crashes in the former Soviet nations than in the U.S., Canada and Mexico.
Q: What do we know about this plane?
A: The Airbus A321-200 was produced in 1997 and operated by Metrojet since 2012. The aircraft had accumulated some 56,000 flight hours in nearly 21,000 flights, meaning that it's typical journey was just over 2 1/2 hours. The A321-200 is the largest version of the highly-popular A320 family of single-aisle jets that can seat up to 240 passengers. The first A321 entered service in January 1994.
At the end of September, there were some 6,500 A320 family jets flying with more than 300 airlines worldwide.
Scott Mayerowitz can be reached at http://twitter.com/GlobeTrotScott.