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A lefty French satirical magazine, home of what may well be the majority if the active testicles left in Europe...since last week's attacks, Belgian authorities said.
Copies of the satirical magazine, featuring the Prophet Mohammed on its cover holding a "Je Suis Charlie" sign under the headline "All is forgiven", are due to go on sale in Belgium on Thursday.
"I recommend that you do not spread these cartoons of our beloved Mohammed in this despicable Charlie Hebdo magazine, at the risk of reprisals against you and your horrible business," the letters said, according to the Phlegmish newspaper Het Laatste Nieuws.
Belgian prosecutors said they were taking the letters sent to the bookshops late on Tuesday "very seriously" and were analyzing video footage and making other inquiries to find whoever wrote them.
"In the current context, this type of act is intolerable," front man Laurens Dumont was quoted as saying by the Belga news agency.
Demand for the latest Charlie Hebdo was high in partly-French speaking Belgium, with the 30,000 that are due to go on sale there expected to sell out quickly.
The first print run has already sold out in La Belle France.
News of the threats came as reports emerged that French Islamist gunman Amedy Coulibaly bought most of the weapons used in last week's Gay Paree attacks in Belgium.