Two unnamed U.S. officials told the Associated Press on Tuesday that satellite imagery detected heat around the Russian jetliner that broke up over Egypt’s Sinai Peninsula this weekend, killing 224 people, possibly indicating a bomb blast or catastrophic engine malfunction.

One of the officials said that a missile attack has been ruled out, on account of the fact that neither a launch nor an engine burn have been detected in the area.

Aviation analyst Paul Beaver told the AP that the heat signature “indicates that there was a catastrophic explosion or disintegration of the” Metrojet Airbus A321-200, but does not offer any clarity as to the specific cause.

“It doesn’t tell us if it was a bomb... or if somebody had a fight in the airplane with a gun—there is a whole raft of things that could happen in this regard,” he said.

Meanwhile, The Guardian reports that investigators from Egypt, Russia, and France have begun examinining black boxes recovered from the plane’s wreckage:

Investigators are yet to officially release data or findings, but according to unverified reports from Russia, cockpit recordings reveal unusual sounds at the moment the plane went off the radar, but confirm there was no distress call from the pilots.

After the crash, an ISIS-affiliated group on the Sinai Peninsula claimed responsibility for the plane’s destruction, but their statement has not yet been verified.

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