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AMSTERDAM (Reuters) - A letter bomb caught fire at a bank in Amsterdam on Thursday and another was disarmed at an office near Utrecht, the latest in a series of such attacks in the Netherlands this week.

No-one was hurt in the incident at an Amsterdam branch of ING Bank, a spokeswoman for the bank said, although police said one person had inhaled smoke.

“It did not explode, it caught fire,” said police spokesman Rob van der Veen.

Police disarmed a letter bomb at an office of U.S. information technology firm Unisys near Utrecht. No one was injured. Police said they had “rendered the letter harmless” and a team from Amsterdam was investigating.

Several letter bombs have been sent to businesses around the Netherlands this year. No injuries have been reported.

Police believe the two letters that detonated harmlessly this week, in mail rooms at Dutch bank ABN Amro and Japanese printer company Ricoh, are connected with letters sent to seemingly random business locations around the country in early January.

Those included two hotels, a gas station, a real estate agent, and a car dealership, among others. None of the earlier letters detonated.

Police say the person sending the dangerous letters is demanding a ransom of undisclosed size to be paid in bitcoin.

In Spain, the Madrid headquarters of U.S. online retailer Amazon.com Inc AMZN.O were evacuated on Thursday after a bomb threat, police said. There was no indication the incident was linked to the Dutch letter bombs.