Dutch police have warned companies to look out for padded envelopes with two stamps. [Source: BBC]

Police are investigating a spate of letter-bomb attacks on companies across the Netherlands.

Four letter bombs have been found in the past two days.

The head office of ING bank in Amsterdam was the latest place to be targeted on Thursday.

Police say the attacks so far have all included blackmail letters demanding payment in Bitcoin. “We need to see if they’re all connected,” a spokeswoman told the BBC.

The attack on ING’s office in the Bijlmer area of south-east Amsterdam was still being investigated but so far no-one has been wounded in the attacks.

An office worker was treated by ambulance staff on Thursday for smoke inhalation.