A United Airlines flight from Rome to Chicago was diverted to Ireland’s Shannon Airport on Monday after a message perceived as a potential security concern was discovered on board, Irish police and airline officials said.

The message made reference to a bomb on the plane, a source with knowledge of the investigation told Reuters.

The 207 passengers and 11 crew members disembarked from the Boeing 767-330 at Shannon Airport at 2:15 p.m. local time, according to Jim Molloy, a representative of Ireland’s police force.

The passengers of flight UA971 were in the process of being searched, he said in an emailed statement, but gave no further details about the nature of the threat.

United said in a statement that the landing was due to “a potential security concern,” but declined to comment further.

A second U.S. government source said similar incidents involving threatening notes found on airplanes happen about once a week.

The handwriting on the note was being analyzed to see if it matched with any other such notes, which are periodically found on aircraft, one of the sources said. It was not clear whether the note was posted in the bathroom prior to takeoff or during the flight.

Police were also taking handwriting samples from the passengers, the Irish Times reported. The message was found in the plane toilet, it said, without citing sources.

The United jet completed its descent after jettisoning some of the fuel loaded for its transatlantic crossing, according to recordings on air traffic control site LiveATC.net.