A plastic explosive could have brought down the Russian airliner that crashed in October over the Egyptian desert killing all 224 people on board, a source in Russia’s Federal Security Service (FSB) told a local newspaper on Tuesday.

The FSB source told Russia’s Kommersant newspaper that the plane was apparently blasted out of the sky over Sinai Peninsula while en route from Sharm el-Sheikh to St. Petersburg by a small bomb built using an unidentified type of plastic explosive.

Russian investigators suspect that the explosive device was planted underneath a passenger seat on the rear right side. A low-energy blast tore a 1-meter (40-inch) hole in the body of the ascending plane, causing it to depressurize.

According to the security source, investigators drew these conclusions after studying traces found on luggage and personal belongings of passengers, and debris of the crashed airliner.

A preliminary report by Egyptian investigators, in charge of the probe, claimed on Monday they had found no clues that pointed to a terror act as the cause of the plane crash. The Kremlin said it disagreed.