In a statement, the Department of Homeland Security said it was taking steps to enhance air security. "Some of these security measures will be visible while others will not," said the statement.
"The public may recognize specific enhancements including heightened cargo screening and additional security at airports. Passengers should continue to expect an unpredictable mix of security layers that include explosives trace detection, advanced imaging technology, canine teams and pat downs, among others. As always, we remind the public to remain vigilant and report suspicious activity to local law enforcement."
A spokeswoman for the Jewish Federation of Chicago told ABC News the federation was alerted this morning and passed on the alert, advising local synagogues to take security precautions. She also said the group was not one of the targets.
The FBI's Chicago office said that the packages "in question" had been addressed to two Chicago locations, but would not provide further details. The FBI "notified the targeted institutions" by phone this morning, according to a spokesman. The White House said that the synagogue across the street from the president's home in Chicago was not one of the synagogues targeted.
The Anti-Defamation League released a statement that said it had learned from "law enforcement sources" of a reported threat to Jewish institutions in packages shipped from the U.K., Yemen and Saudi Arabia, and that it had sent notice to Jewish organizations across the U.S.
While President Obama did not say that Al Qaeda in the Arabian Peninsula was behind the apparent bombing attempt, he did note in his afternoon address that Al Qaeda's Yemeni offshoot continues to seek ways to attack the United States.
The radical American-born cleric Anwar a-Awlaki has repeatedly called on for attacks on the U.S. from his base in Yemen.
American officials believe that AQAP has trained hundreds of terrorists whose prime objective is to attack the US, including the so-called underwear bomber Umar Farouk Abdulmutallab.
"Yemen is the home of between 500 and 600 al Qaeda fighters, three or four times the number of al Qaeda fighters in Afghanistan," said Richard Clarke, former White House counterterrorism official and now an ABC News consultant. "This is really a place where Al Qaeda is operating, where it's strong, where it has training camps, where we know they have trained people and sent them to the United States."
U.S. authorities have now ordered every package shipped from Yemen to the U.S. to be inspected.
FedEx also said in a statement that local authorities, in cooperation with the FBI, had "confiscated a suspicious package at the FedEx facility in Dubai. The shipment originated in Yemen and as an additional safety measure, FedEx has embargoed all shipments originating from Yemen."
The method of trying to ship the improvised explosives into the country on air cargo flights exposed one of the worst-kept secrets about American security.
At best, only a third of packages coming into the country as air cargo are ever inspected.