An Egyptair plane from Cairo to New York has landed at Glasgow Prestwick Airport after being diverted.
BBC producer Nada Tawfik, a passenger on board, found a note in the plane's toilet with the words "I'll set this plane on fire" and a seat number.
The Boeing 777 - which was travelling to JFK International Airport - touched down at around 14:30 BST.
Air traffic control said action was taken following "a problem", and that standard procedure had been followed.
An eyewitness told the BBC that he saw two RAF Typhoon fighter jets escort the plane into the airport, in Ayrshire.
The aircraft is carrying 326 passengers, who are starting to disembark after being on board for more than three hours after it landed.
Police are waiting to interview them.
'Pretty tense'"Start Quote
End Quote Nada Tawfik Passenger and BBC producerIt almost looked like a child's handwriting or someone who has very sloppy handwriting"
Ms Tawfik, who was travelling with her young daughter, said she found the note - written on a napkin with a pencil - inside the sink about three hours into the flight.
She immediately reported it to cabin crew who then locked the toilet.
"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," she said.
"I said to one of the stewardesses 'I don't know if this is a prank', they said no, it can't be a prank.
"Either someone has a very bad sense of humour or, you know, it's very scary.
"As you can imagine, it's a pretty tense situation.
"Everyone's trying to grab what they can to drink, people are concerned, there are babies on board who are all getting very frustrated."
The plane has been moved to an isolated part of the airport, away from the passenger terminal.
The BBC's James Shaw, who is outside the airport, said the plane was thought to have been surrounded by between 10 and 15 police vehicles, and fire engines were on stand-by.
BBC producer Nada Tawfik said she initially thought the note was a prank
Police Scotland confirmed a suspicious note was found on the plane and will interview all passengers on board.
A spokeswoman said: "At around 1420 hours today an Egyptair aircraft flying from Cairo to New York was diverted to Prestwick Airport after a suspicious note was discovered on the aircraft.
"There are no reports of anyone injured."
A statement on the airport's website said there were no disruption to flights, which were operating as normal.
The A79 is closed in both directions next to the airport between the B739 Station Road junction and the Shawfarm Road junction.
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