The incident happened Saturday as the Egypt Air flight was en route to New York City from Cairo, Egypt's State Information Service reported.
"At around 1420hrs yesterday (13:20GMT) an Egypt Air aircraft flying from Cairo to New York was diverted to Prestwick Airport after a suspicious note was discovered on the aircraft," said a statement from Scotland police.
At the airport, about 100 passengers were let off the plane and "taken to the airport terminal where they are being looked after by aircraft staff," the statement said. All passengers were questioned Saturday evening. There were no reports of injury.
Carol Gable, an NBC News producer who was on board the, said an Egypt Air flight attendant told her that the note found was "carefully and legibly" written in English and that it made reference to a fire, "not about a bomb," Gable told NBC News.
BBC producer Nada Tawfik was also aboard the plane and told the BBC that she saw a note with the words "I'll set this plane on fire" and a seat number.
"It almost looked like a child's handwriting or someone who has very sloppy handwriting, but it was very alarming especially these days when everyone is so concerned about safety on flights," Tawfik said, adding that the ordeal was "very scary."