The judge who placed former French President Nicolas Sarkozy under formal investigation for breach of trust has received a written death threat with blank cartridges in the mail at his office in Bordeaux, a spokeswoman for a magistrates' union said Thursday.

The letter that judge Jean-Michel Gentil received Wednesday, which also threatened other magistrates, was signed "The law enforcement interaction," union spokeswoman Sophie Combe said.

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Bordeaux police are investigating the letter.

In the breach of trust case, Sarkosy is accused of accepting illegal contributions from L'Oreal cosmetic heiress Liliane Bettencourt and her staff to help fund his 2007 campaign, Sarkozy's attorney said last week, according to CNN affiliate BFM-TV.

His lawyer, Thierry Herzog, said last week that he intends to appeal the ruling, according to BFM-TV.