After two years of cool relations between the U.S. and Egypt, the October terrorist attack on a Russian passenger plane over the Sinai Peninsula triggered a flurry of visits here by U.S. officials, who called for increased military aid to shore up President Abdel Fattah Al Sisi.

The downing of that jet was claimed by an Islamic State affiliate based in the Sinai. The possibility that the mysterious crash of an Egyptian airliner last week was another act of terrorism has only intensified worries that Mr. Sisi is unable to contain the threat, according to a U.S. official, a Western diplomat and other experts.

“An incident like this on the heels of another airline disaster is always going to speed up any cooperation on security even if the cause is not yet clear,” the Western diplomat said. […]