Jewish Institutions in United States on Alert

The Anti-Defamation League has sent notices to Jewish institutions across the United States to step up mailroom security and to contact law enforcement immediately if officials or members see anything suspicious.

The action came in light of the developments Friday in which packages containing explosive materials originating in Yemen were "bound for the United States -- specifically, two places of Jewish worship in Chicago, said President Barack Obama.

Based on this "credible terrorist threat," as the president called it, "all churches, synagogues and mosques in the Chicago area should be vigilant for any unsolicited or unexpected packages, especially those originating from overseas locations," the FBI said in a statement.