Egypt said on Thursday that explosive traces had been detected on the bodies of passengers retrieved from EgyptAir Flight 804, which plunged into the Mediterranean Sea in May and killed all 66 people on board.

The announcement by the Civil Aviation Ministry offers the strongest suggestion yet that a bomb might have felled the airliner as it flew to Cairo from Paris. Previously, officials had focused on a fire as a likely cause.

Still, it was not clear why Egyptian officials had taken so long to draw the conclusion about explosives — most of the bodies were recovered from the sea by July — and experts said the cause of the crash remains a mystery.

“The timing is odd, and the results are very late,” said Shaker Kelada, a former chief aviation investigator with the ministry.