The crash of a Russian passenger jet that broke up mid-flight is not thought to be due to a technical malfunction or pilot error, the airline said Monday. The incident is mysterious and may lead to investigations of an attack.

The Washington Post reports that the latest statements from Moscow have not given clear directions of the investigation into the crash that occurred Saturday, killing all 224 passengers aboard.

There are suspicions that the incident may have been terrorist linked due to the militant factions in the region, like the Islamic State. The group is a target of Moscow military intervention efforts.

Officials for the airline say, however, that the incident could only be due to a “mechanical impact on the aircraft.” A spokesperson for Russian President Vladimir Putin has said that terrorism has not yet been ruled out, and urged for patience as the investigation continues.

Airline officials say that the crash could not have occurred due to a fuel problem or engine failure. Neither of those would have caused the plane to separate in midair. Officials have not commented further on what they mean by an “impact” to the aircraft.

The statements have drawn attention to a claim made by the Islamic State that they brought down the aircraft, which was mostly ignored by Russian officials, but caused some international flights to reroute themselves.

Many defense experts doubt that the Islamic State possesses missiles that are able to hit an aircraft flying at 31,000 feet. The director of National Intelligence, in Washington says that there is no “direct evidence” of terrorist involvement in the incident, but it has not been fully ruled out yet.

Russia’s Prime Minister has called for a comprehensive investigation of the crash, with the search for wreckage likely ending Monday. Experts from the airline, Russia, and Egypt will examine what is recovered.