Hundreds of false bomb threats that were emailed over the weekend to Jewish facilities across the US are believed to be connected and originating from outside the country, according to the FBI.

“At this time, based on similar language and specific email tradecraft used, it appears the perpetrators of these threats are connected. Additionally, these threats appear to be originating from outside of the United States,” the FBI said in a message to national Jewish organizations that was obtained by CNN.

“To date, none of these email threats have involved any actual explosive devices or credible risk of harm to congregants,” the bureau added.

More than 400 Jewish facilities received the false bomb threats over email since Saturday, the Anti-Defamation League, an international Jewish non-profit organization, said on Monday.