At least six companies in Amsterdam, Rotterdam and Utrecht have been sent letter bombs in the past few days and police are warning that there will probably be more to come.

In all cases so far, the letter bombs failed to go off, but they could have caused serious injury, policy say.

The recipients include an estate agents in Utrecht, petrol stations in Amsterdam and Rotterdam and a Mercedes dealership in Rotterdam. On Friday morning the Okura hotel in Amsterdam reported it had received a similar letter.

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 and that it too has received one of the letter bombs.

As yet, it is unclear what connects the companies and specialists from the Dutch forensics institute are examining the packages.