French Prime Minister Jean-Pierre
Raffarin has received a letter from an unknown Muslim group that threatened to attack France, French officials say.
Mr Raffarin reportedly received the threats via Le Parisien newspaper
A group calling itself Servants of Allah, the Powerful and Wise One sent the message via Le Parisien newspaper.
Anti-terrorism police have taken on the official inquiry into the threats against France and French interests abroad, the justice ministry stated.
The threats came five days after train bombs in Madrid killed 201 people.
"It is obviously not yet possible to assess the worth of this message," said the justice ministry in a statement.
Interior Minister Nicolas Sarkozy told Agence France Presse news agency that experts were analysing the threats.
The two-page letter was delivered to the newspaper's offices early on Tuesday, sources close to the investigation told AFP.
Both Le Parisien and Le Monde said the letter was signed "Mosvar Barayev commando".
The signature was similar to Movsar Barayev, the leader of a Chechen group that took hundreds hostage in a Moscow theatre in October 2002 - and died during the siege.
Le Parisien's news director Christian de Villeneuve told AFP that the letter "threatens France with reprisal attacks" following the adoption earlier this month of the law banning the wearing of headscarves in state schools.