Four more threatening letters have been sent to business addresses in Amsterdam and Leusden, near Utrecht, bringing the total to 15.

The letters contained small quantities of an unnamed powder and appear to be linked to 11 packages that were delivered to hospitals, hotels and a newspaper office on Tuesday. Nobody was injured, police said.

One of Wednesday’s packages was sent to a hospital in Amsterdam. Police said they were following up a number of leads.

Police have not disclosed what type of powder the letters contained, but said the amount was not enough to cause harm.

In February a number of companies in the Netherlands, including several hotels in Amsterdam, were targeted in a letter bombing campaign. Police said they do not yet know if this week’s attacks are connected.