Threatening letters containing white powder have been send to two new locations in the Netherlands, news website Nu.nl said on Thursday.

One letter was sent to an address on the Prinsengracht in central Amsterdam, another to a hospital in Roermond. The new sendings take the total number of threatening letters currently known about to 17.

Police would not say which building in Amsterdam was targeted in the latest mailing. One letter is now known to have contained a harmless quantity of pesticide, but police have also declined to give more details, website Nu.nl reported.

‘We advise people not to open suspicious letters and to call the police,’ a spokesman told the website. ‘If you do discover powder in an envelope, wash your hands and don’t touch it.’