ZurichZurich airport has re-opened after it was partly closed for several hours on Monday evening because of an unspecified threat, authorities said.

The airport shut Check-in 1 and 2 as well as Arrivals 2 and passengers were re-directed to Check-in 3, Zurich’s cantonal police said in a statement. Around a dozen flights were delayed by up to 30 minutes.

The closed-off areas were re-opened shortly before 1:00 a.m (0000 GMT) after police had determined no dangerous objects were in the area, police said.

Several local media outlets reported authorities had received an anonymous tip about a bomb.

The police did not give details about the threat and said its investigation was ongoing.

(Reporting by Caroline Copley; Editing by Larry King)