Neither package ever made it to the targeted institutions, but the two intercepted mail bombs have prompted a quick and determined response, both in Chicago and around the world.
The near-miss shows that the suspected al-Qaida bomb was sophisticated enough to escape notice. It also shows how close terrorists came to getting the explosives airborne and bound for the U.S.
Intelligence officials were tipped off to a pair of explosive packages on planes in England and the United Arab Emirates early Friday morning.
After a six-hour sweep of cargo at the East Midlands airport in central England, Leicestershire police came up empty and removed the security perimeter they had set up, British aviation safety consultant Chris Yates said.
But when officials in Dubai said they had discovered a bomb disguised as a computer printer cartridge, authorities urged the British to look again, a U.S. official said, speaking on condition of anonymity because he was not authorized to discuss the matter.
"As a direct consequence, they put the cordon back up again and looked again and found the explosives," said Yates, relying on a report given to him by an eyewitness to the searches.
President Barack Obama's counterterrorism adviser, John Brennan, called it "a very sophisticated device, in terms of how it was constructed, how it was concealed."
"It was a viable device. It was self-contained, so it could have been detonated and activated," Brennan told NBC's "Meet the Press," adding that officials are trying to determine whether the planes or the synagogues were the intended targets.
On Friday evening, the Leicestershire police handed control of the investigation to the Metropolitan Police, the London-based agency also known as Scotland Yard.
Leicestershire police declined to answer questions Sunday about the searches, referring callers to Scotland Yard, which traditionally takes the lead in major terrorism cases in Britain.nsion of international mail from Yemen, which is believed to be the origin of the cargo bombs, as authorities made at least one arrest and revealed that the terrorists used forged documents and ID cards in the foiled plot.
With extra security present, many of those in Chicago's Jewish community continued their Sabbath, despite word at least one Jewish house of worship may have been the target of a foiled terrorist plot.
ABC7 Chicago confirmed Or Chadash in Edgewater was to be one of the recipients. The small Jewish congregation serves a mainly gay, lesbian, bisexual and transgender community and has about 100 members.
"We don't know how we were located. We don't know if it was local or foreign. We just don't know, and at this point, most of us are feeling that we'd rather not make ourselves personal targets," the co-president told ABC7.
Or Chadash shares facilities with the Emanuel Congregation. The rabbi of that group offered a possible explanation for why they may have been targeted.
"Maybe someone actually knew that and thought, 'Ah, here's a way to get both Jews and homosexuals,'" said Emanuel Congregation's Rabbi Michael Zedek.
"I feel that a threat to a synagogue is a threat to any religious institution in this country. The terrorists that we're dealing with are really attacking the Western culture and the Western world," said Rabbi Michael Siegel of the Anshe Emet Synagogue.
Siegel leads Chicago's Anshe Emet congregation. He said an FBI agent visited his home Friday night to discuss the bomb plot. Siegel tells ABC7 Chicago there is no indication his was the second synagogue that was targeted.
The bombs were discovered Friday after one had been shipped by Fedex through Dubai and the by UPS via England.
"I can confirm the device was viable and could have exploded," said British Home Secretary Theresa May.
Security officials in England say the powder hidden in a toner cartridge inside a shipped computer printer may be the highly explosive PETN, the same used in the failed plots of the so-called 'shoe and underwear bombers.'
Last month, U.S. intelligence officials warned that terrorists may try to mail chemical and biological materials as a part of an attack on the United States and other Western countries. Synagogues in the Chicago area were advised Saturday to take appropriate precautions.
The Associated Press contributed to this report.