Passengers disembarked 40 minute later.
A US-bound flight that was redirected to Shannon Airport on Monday afternoon following the discovery of a suspicious note on board, has been handed back to the airline following extensive searches.
"Gardaí attended an incident at Shannon Airport yesterday 11th June 2018 at approximately 12.30pm after receiving a report of a suspicious device allegedly on board a plane", they said.
The airline had said on Monday: "United flight UA971 en route to Chicago from Rome, diverted to Shannon following a potential security concern".
The airline confirmed the flight diverted because of a "potential security concern", declining to comment further.
No further details were made available.
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The Irish Times reported, without citing its source, that a message had been discovered in the plane's toilet claiming that there was a bomb on board.
"We will work to get customers on their way to Chicago as quickly as possible", United said in a statement.
The airline said on Twitter that the flight was now canceled and would depart for Chicago on Tuesday.
According to the Irish Times, gardai are taking handwriting samples from passengers to try and establish who is behind the note.
They also confirmed that the plane and all the passengers are now being searched.