CAIRO—A Moscow-bound EgyptAir flight returned to the Egyptian capital shortly after takeoff Wednesday after a threatening message was found onboard, authorities said. A statement by Egypt’s flagship airline said flight MS79 had returned safely to Cairo International Airport some 22 minutes after takeoff. The flight was heading to the Russian capital, it said. Citing an unnamed official, the statement said the written message was found on a seat of the Airbus A220-300. It did not elaborate on the content of the message or who wrote it. Later on Wednesday, EgyptAir said a security inspection determined that there was no threat to the flight. It said another aircraft, a Boeing 737-800, flew the 93 passengers to Moscow. All flights between Moscow and Cairo were suspended for 2 1/2 years after the local affiliate of the ISIS group downed a plane over Egypt’s Sinai Peninsula in October 2015, shortly after the aircraft …