The cause of the weekend crash of a Russian passenger airliner in the Sinai Peninsula, killing 224 people on board, remains unknown, but the suggestion that ISIS smuggled a bomb on board may be getting a boost today, with officials confirming a military satellite detected a mid-air heat flash from the plane before the crash.

The heat flash suggests some sort of explosion on board the plane, which could have been an accident, but also lends credence to ISIS’ claim of responsibility for downing the plane. Officials have yet to determine exactly how the plane crashed, and say it could take weeks.

Egyptian and Russian officials have been trying to downplay the chances of a bombing, but have offered no really good alternative explanation yet, while the Russian airline company has insisted there were no signs of technical failure or human error.

It is puzzling that the “heat flash” is only coming out now, four days after the crash, and experts say that if they’d swabbed the debris shortly after the crash they’d have been more readily able to identify explosive residue. This once again is likely to take weeks to sort out.