Maxim Shemetov / Reuters

Russian families have begun identifying the victims of Saturday’s plane crash in Egypt, which killed all 224 people on board.

Egypt’s aviation minister said Tuesday that Egyptian and Russian investigators are starting to examine the black boxes—devices that record flight data and audio from inside the pilot’s cockpit—recovered from the downed commercial jet. But officials from both countries, as well as aviation experts, say it’s too soon to know what caused the crash.

There is no consensus over the events preceding the crash, either. On Sunday, a Russian aviation official said that because debris was spread over a large area, the plane must have broken apart in midair. On Tuesday, Egypt’s civil aviation ministry said there was no evidence to back up that claim.

Flight 9268, operated by Russian airliner Kogalymavia under the name Metrojet, departed Sharm el-Sheikh, a resort city on the Red Sea, early Saturday morning, bound for St. Petersburg. All the passengers and crew were Russian, except for three Ukrainians and one Belarusian. About 20 minutes into the flight, the plane dropped off radar screens. Egyptian rescue teams discovered the wreckage in a remote part of the Sinai Peninsula. Russia has established a state commission to investigate the crash, and other nations are also aiding in investigations.

The black boxes may hold the answer to what happened to Flight 9268. But until their contents are analyzed and released, people are left to speculate. Here’s a guide to some of the theories about what may have happened.

Technical Problems

Russian officials have not yet ruled out the possibility of a technical malfunction. Egyptian officials say the plane passed a pre-takeoff inspection, and Kogalymavia has insisted the Airbus plane was in “excellent condition.” But the wife of the co-pilot told a Russian TV channel on Saturday that her husband, speaking to a daughter, “complained before the flight that the technical condition of the aircraft left much to be desired.”

A CNN aviation analyst, formerly of the National Transportation Safety Board, considered on Tuesday the role of “an earlier maintenance problem.” In 2001, the tail of the same Airbus that crashed on Saturday was damaged when the aircraft struck the tarmac upon landing in Cairo. But Kogalymavia said Monday the tail had been “fully repaired,” and so could not have contributed to the crash. Reuters points out that poor tail repairs have caused two major civilian aviation accidents, one in 1985 that killed 520 people and another in 2002 that killed 225. In those cases, the repairs had been made years before the planes crashed.

There are conflicting reports about whether the pilot in Saturday’s crash communicated technical difficulties to air controllers before losing contact. One Egyptian aviation official said the pilot radioed to say he would attempt to land at the nearest airport, another such official said “there was nothing abnormal” before the crash.

‘Mechanical Force’

A Kogalymavia official said Monday that the crash was not the result of technical problems, but of another “technical or physical action” from outside of the aircraft.

“The only possible explanation is a mechanical force acting on the aircraft,” said Alexander Smirnov, the company’s deputy general director, though he didn’t say what that “force” could be.“There is no combination of system failures that could have broken the plane apart in the air.”

He added: “We rule out a technical fault and any mistake by the crew.”

‘A Catastrophic In-Flight Event’

Citing a U.S. official, CNN reported Tuesday that a U.S. military satellite detected a heat flash in the sky over the Sinai that officials have attributed to Flight 9268. The presence of a heat flash suggests that there was “a catastrophic in-flight event”—possibly an explosion from inside the plane, CNN wrote. More:

Analysts say heat flashes could be tied to a range of possibilities: a missile firing, a bomb blast, a malfunctioning engine exploding, a structural problem causing a fire on the plane or wreckage hitting the ground.

Terrorism

A local affiliate of the Islamic State quickly took responsibility for the crash, saying that it “brought down” the plane, but did not offer any evidence. Russian officials have dismissed that claim, but haven’t ruled out terrorism as a possible cause. NBC News reports that U.S. intelligence analysts say satellite imagery rules out missiles as the cause.

Miscellaneous ‘Elements’

Russian news agency TASS, citing unnamed sources in Cairo, reported Tuesday that investigators had found and will analyze mysterious “elements” at the crash site that are not typically found in an Airbus’s structure. But the sources also said the material may not be sinister at all, and could be diving equipment from passengers’ luggage. NBC News said an adviser to Egypt’s aviation minister has denied the TASS report.

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