DUBLIN - A United Airlines plane that was diverted to Ireland's Shannon Airport on Monday after a security alert has been returned to the airline after nothing suspicious was found on board, the Irish police said.

A message found on the plane referred to a bomb, a source with knowledge of the investigation told Reuters. The United jet landed once it had jettisoned some of the fuel loaded for Rome-Chicago flight.

"Searches have been completed and the plane had been handed back to the airline," a police spokesman said on Tuesday. "Nothing was found."

(Reporting by Conor Humphries, editing by Larry King) ((conor.humphries@thomsonreuters.com; +35315001518; Reuters Messaging: conor.humphries.thomsonreuters.com@reuters.net))