United Airlines flight from Rome to Chicago diverted to Shannon, Ireland due to a "potential security concern," the airline said Monday.

Local police said Flight 971 landed at Shannon at 2:15 p.m. local time, after a "message" was discovered. The plane and passengers were being searched, police said.

A source familiar with the incident said a threatening note was found on board the plane, a Boeing 767-300. United declined to comment on the reason the flight diverted. The flight had 207 passengers and 11 crew members on board, United said.

The airline "will work to get customers on their way to Chicago as quickly as possible," United said in a statement.

Passengers will return to Chicago on Tuesday and the airline is working to help travelers with overnight accommodations after the flight was canceled, United said on Twitter.

Hello, Don. UA 971 canceled in Shannon. Customers and crew will be receive assistance with overnight accommodations and will arrive into Chicago on June 12th. Rebooking for customers with connections beyond Chicago is currently underway. ^TY