A leading Jewish human rights organization called on Attorney General Jeff Sessions to create a special task force to investigate the bomb threats targeting Jewish Community Centers around the nation.

In a letter sent Monday, Rabbi Marvin Hier and Rabbi Abraham Cooper, dean and associate dean of the Simon Wiesenthal Center, wrote of their growing alarm over the waves of threats since January. They urged Sessions to establish a task force with the assignment of identifying and capturing the culprit or culprits who are now terrorizing American Jewry.

At least 11 JCCs in 10 states received threatening phone calls on Monday. It was the fourth series of such messages since the start of the new year, raising the total number of incidents to about 70 aimed at almost 60 JCCs in 27 states and one Canadian province.

Anti-Semitic hate crimes comprise the largest portion of religiously motivated attacks in the United States. The rabbis, pointing to a supercharged anti-Semitism spreading on the internet and social media, wrote that they are deeply worried that the bomb threats could inspire more dangerous targeting of the Jewish community.