Two more companies in Amsterdam have received threatening letters connected to the spate of letter bombs sent to at least six different companies nationwide, the Parool said on Monday.

A garage and another unknown firm received the threatening letters this weekend, the paper said. The contents are said to be ‘in line’ with the previous letters sent to a variety of companies. Neither have had a letter bomb.

The Dutch forensic institute is currently examining all the letters in an effort to track down the sender.

Last week,companies in Amsterdam, including the Okura and Victoria hotels, an estate agents in Utrecht and a petrol station in Rotterdam were among the recipients.

The envelopes have a sticker indicating they come from CIB, a bailiff’s company based in Rotterdam, but police say the company is not responsible.

In total, police are now aware of 10 threatening letters and six actual letter bombs containing explosives.