More than a hundred bomb threats made to Jewish groups in the U.S. and U.K. since Jan. 7 are believed to be linked, investigators said Thursday. Scotland Yard and the Federal Bureau of Investigation are collaborating to probe the threats, which have reportedly been made over the phone by men and women with American accents using an electronic scrambler to distort their voices. At least 140 such threats have been made to Jewish centers in the U.S. alone, Jewish community leaders told Reuters. Although the callers claim there is an improvised explosive outside the building in each call, no bombs have been found in either country’s facilities. Jewish community centers, schools, and national organizations in numerous cities have been bombarded with the menacing phone calls, often prompting evacuations. Hours later, those phone calls have been followed by similar threats against organizations in Britain, Jewish community leaders said. Although Manhattan prosecutors say former journalist Juan Thompson was behind at least eight phoned-in threats in the U.S., Jewish community leaders do not think Thompson’s alleged threats were part of the wider campaign against Jewish organizations.