SAN DIEGO—Six San Diego-area synagogues were among scores of Jewish religious facilities statewide whose daily operations were disrupted Jan. 2 by false bomb threats, authorities reported.

The centers of worship were targeted by a group email sent to a total of 91 facilities across California shortly before 7 a.m., said Bill Ganley, community security director for the Jewish Federation of San Diego.

The note stated that explosives had been planted at the various sites and would detonate “soon,” according to Mr. Ganley and Lt. Zheath Sanchez of the San Diego County Sheriff’s Department.

Law enforcement searches of the temples turned up no reported hazards.

The local institutions victimized by the hoax were Congregation Beth Israel in San Diego, Congregation Etz Chaim in Ramona, Temple Adat Shalom in Poway, Temple Emanu-El in San Diego, Temple Etz Rimon in Carlsbad, and Temple Solel in Encinitas.

The FBI has taken charge of the case, Mr. Ganley said. No suspects had been publicly identified as of Tuesday evening.

Such antisemitic terrorist threats have been on the rise, Mr. Ganley noted. Between Dec. 13 and New Year’s Day, more than 700 were reported to the Secure Community Network, a Jewish security organization that works with hundreds of synagogues across the United States, he said.