A letter containing bullets and death threats to Jews was sent to a synagogue in a northern Paris suburb, where an infamous transit camp for Jews was set up during World War II.
The letter to the synagogue in Drancy also bore a swastika, said Sammy Ghozlan, head of a national body monitoring anti-Semitism.
He said the letter had been sent on August 14 and discovered on Tuesday after the summer break.
He urged authorities to "beef up security in all places of worship" ahead of the Jewish High Holidays, Rosh Hashana and Yom Kippur next month.
Ghozlan said a similar letter had been sent to a synagogue in Stains, another northern suburb.
Drancy was home to an internment camp set up by the collaborationist Vichy regime during World War II. About 65,000 Jews were deported from there to extermination camps.
In 1976, a memorial was set up in Drancy. Since then, the site has been desecrated at least twice with swastikas and anti-Jewish graffiti.