President Obama called the discovery of the devices a "credible terrorist threat."
Those devices, which were shipped by the same person from Yemen, were found on cargo jets Friday. Officials have not said to which synagogues the explosive devices, which are about the size of a breadbox, were addressed. However, they did say neither was addressed to the synagogue across the street from President Obama's Hyde Park home, where he is expected to spend part of the weekend while in Chicago.
"Since two of the suspicious packages that were intercepted were addressed to religious institutions in Chicago, all churches, synagogues and mosques in the Chicago area should be vigilant for any unsolicited or unexpected packages, especially those originating from overseas locations," said Special Agent Ross Rice, Chicago FBI.
A tip from the Saudi intelligence agency prevented the package bombs from being delivered to two Chicago synagogues and has prompted a concerted search for others.
"We don't want to presume that we know the bounds of this plot," said John Brennan, Homeland Security advisor.
Officials said they believe the explosive PETN was packed in the envelopes found in England and Dubai. The mail bomb packages, officials said, would have detonated upon opening and caused significant destruction and probably death.
PETN is the same nitroglycerin-style chemical powder that was used in the underwear bomber attempt on a Detroit-bound flight on December 25, 2009, and the shoe bomber attempt on a flight in 2001.
Several cargo planes and trucks were searched Friday after the discovery. It's not clear if more packages were found and to whom they were addressed. Linda Haase, associated vice president of Jewish Federation of Metropolitan Chicago, confirms synagogues were notified to be on alert, but would not release details.
"Law enforcement told us to give these types of recommendations and right now that is limited to packages. And I think if law enforcement were concerned about different type of stuff, they would have given us that information," said Clare Pinkert, Anti- Defamation League.
Extra security was noticeable at some synagogues in the Chicago area Friday night. Chicago police say they have increased patrols near several synagogues, including those in the West Rogers Park neighborhood where evening services were well attended.
"It's a show of strength that we're going to stand up to this," said Alan Gilbert, synagogue member.
While many synagogue officials told ABC7 they were heeding the warning about deliveries, some, including Temple Shalom on the city's North Side, continued receiving packages.
"I'm very proud of the coalition of law enforcement officials with whom we work very closely on a regular basis who are absolutely certain that they can, that we can deny that we were one of the intended recipients," said Rabbi Aaron M. Petuchowski, Temple Shalom congregational leader.
Chicago FBI officials said other than the addresses on the packages, there are no identifiable or specific threats to the Chicago area. Even before Friday's attack was derailed, the agents were concerned al-Qaida was trying to recruit men in the U.S.
"Obviously of big concern to everybody in the law enforcement community and the intelligence community is another 9-11 style attack in which thousands of people were killed. At the same time are equally important to us right now, um, are attacks of a smaller scale. These are the kinds of attacks that Inspire magazine is suggesting and they'll go as far as to say, um, this particular attack may result in the death of 10 people or 50 people, the numbers are smaller than what we experienced with the 9-11 attacks but none the less they're terrorist attacks," said Special Agent Bill Monroe, Chicago FBI.
An online magazine that al-Qaida operatives in Yemen published used Chicago as a backdrop for a new attack strategy where radical Islamists would strike innocents on busy streets.
"Their view, and it's a distorted view, is that the United States was responsible for the death of, I'll use the term innocent individuals, in other areas of the world, so in their mind everybody in the United States is fair game," said Special Agent Bill Monroe.
The ABC7 I-Team learned the packages would have come through O'Hare International Airport.
Despite the threat- with Chicago air freight at the center of it- O'Hare Security Chief Richard Edgeworth is heading to Israel for a security conference and Aviation Commissioner Rosemary Andolino plans to leave Saturday for Amsterdam to attend a conference on cargo development.
A spokesperson for the Chicago Department of Aviation insists that numerous top officials including veteran deputy commissioners are fully capable of insuring the operational safety of O'Hare Airport. The spokesperson tells ABC7 that Andolino and Edgeworth are both in touch with Chicago aviation operations while out of the country, by email and cell phone.