Staff at the media complex in Hilversum were advised not to go outside on Monday morning while police investigated a “suspicious” incident.

The Midden-Nederland police division would not disclose details, but said it had stepped up surveillance in the area, with extra officers on duty and a helicopter.

News broadcaster NOS closed the public entrance and activated the night security setting, meaning only the reception can open the gates. Workers were told to enter the building by the staff entrance.

News website AD.nl quoted various sources saying people working on the site had been told to remain indoors, but police said they were free to leave if they wished.

“We are carrying out extra surveillance and will do our best to clarify the situation,” a spokesman said.