The package was then shipped on a different Qatar Airways plane to Dubai in the United Arab Emirates, where the bomb was discovered by authorities late Thursday or early Friday. A bomb of similar fashion turned up in central England on Friday, after authorities there were alerted to the Dubai discovery.
U.S. officials now believe the UPS and FedEx planes were the real targets.
At first, investigators believed the synagogues were the intended targets, but ABC News has learned the packages were sent to addresses that were eight years out of date.
Analysis of the bombs also shows they included cell phone circuitry that would have made it possible to set them off in flight.
Meanwhile, Yemeni officials have released the female engineering student arrested Saturday in connection with plot.
They say someone else used her name to ship the bombs
/*President Barack Obama*/'s counterterrorism adviser John Brennan said that authorities "have to presume" there will probably be other such bombs in existence.
Authorities have said the explosive devices bore the hall hallmarks of /*Al Qaeda*/ and the terrorist group's Yemen offshoot is suspected of mailing the bombs. After a frenzied day of searches in Philadelphia, Newark, N.J., and New York City, no explosives were found inside the United States. All new cargo coming from Yemen has been temporarily banned.
In Yemen on Sunday, police searched for additional suspects after arresting the 22-year-old female computer engineering student suspected of mailing the packages and also detaining her mother. Both arrests were on Saturday.
U.S. officials said suspects in the plot include the bombmaker accused of designing the explosive used in the failed Christmas airliner bombing.
Authorities were also looking at two language institutions in Yemen the plotters may have been associated with.
Also, U.S. inspectors from the /*Transportation Security Administration*/ are heading to Yemen on Sunday to monitor cargo security and pinpoint possible holes in the system.
Following the foiled mailed bombs, the /*TSA*/ has stepped up security using a new method for pat downs in airports. TSA agents will now be able to touch body parts that were once off limits, like a woman's bra or the inner thigh. TSA officials say this new method aims to stay ahead of new and evolving terror threats.
Passengers will only be subjected to these new pat downs if the electronic screening method sends off a suspicious alarm, notifying that they have metal on them.
The Associated Press contributed to this report.