Dutch police were puzzled Friday by a spate of letter bombs sent to seemingly random targets in three of the Netherlands' main cities as it warned the public to be on the lookout. The latest incident happened early Friday when staff at Amsterdam's Okura hotel alerted police after they received a suspect parcel bearing the hallmarks of a letter bomb.

Explosive experts rushed to the luxury hotel and police later tweeted: "The letter is the same as the other ones." "The hotel does not have to be evacuated, but the military's explosives clearing team is on the scene." The letter is one of at least seven that have been sent to various addresses in the three cities in the past week, including two hotels, two service stations, an estate agency, a car dealership and the central debt collecting agency. The director of the car dealership in Rotterdam told NOS public news a receptionist Thursday opened the letter and saw "something that looked like a battery" before calling the police.

So far, the letters "caused no damage, because they didn't explode, but if they did, could have caused serious injury," police said in a statement.