Washington (CNN) -- Additional details surfaced Thursday pointing to similarities between the bombs found in packages mailed from Yemen last week and a failed attempt to blow up a plane as it landed in Detroit, Michigan, last year.
Investigators have found syringes associated with the explosives found last week, a U.S. official said.
FBI agents investigating the failed attack on the plane landing in Detroit on December 25 found what appeared to be the remnants of a syringe.
They made the discovery near the seat of Umar Farouk AbdulMutallab, a 25-year-old Nigerian citizen who faces charges including attempted murder. Investigators say he tried to blow up a plane with an explosive that was partly sewn into his underwear.
Experts have said that the bomber probably used a liquid in a syringe with a chemical he was carrying; that would have created a fire and ignited the explosives.
It is the latest connection to be found between both incidents. PETN, a powerful explosive, was used in the parcel bomb plot and last year's foiled bombing. And officials believe that al Qaeda in the Arabian Peninsula was responsible for both plots.
"This was AQAP," the U.S. official said Thursday. "The body of information that people have reviewed over the past several days points very directly to AQAP.
"AQAP has shown a strong interest, and regrettably skill, in dealing with PETN, but that wouldn't be the only reason why this government would reach the conclusion that they are responsible. There is other information, too."
Homeland Security Secretary Janet Napolitano recently said the two devices found on freight planes in the United Arab Emirates and the United Kingdom last week have "all the hallmarks" of a plot by al Qaeda in the Arabian Peninsula, the Yemen-based arm of the terrorist network. The same group also claimed responsibility for the December bombing attempt.
The most recent security threat came to light Friday, when authorities in the United Arab Emirates and Britain found two packages sent from Yemen with explosives that were addressed to synagogues in Chicago, Illinois.
The devices were packed in computer printer toner cartridges and designed to be detonated by a cell phone, a source close to the investigation said.
The detonating substance was lead azide, a "very powerful initiator" that is easily prepared and is a standard substance in detonations, a source closely involved in the investigation said.
The U.S. official said that testing on the devices is ongoing, but the assumption at this point is that the lead azide in the syringe was supposed to be an improvised blasting cap.
When the circuit was completed, a wire in the syringe would heat up and ignite the lead azide, the official said. That would set off the PETN.
One of the parcel bombs was disarmed minutes before it was due to explode, French Interior Minister Brice Hortefeux told French TV on Thursday in Paris.
"One parcel has been defused, one of the parcel bombs, was defused only 17 minutes before the time of the blast," Hotefeux said on France 2.
The White House and other U.S. officials are not confirming the French minister's assertion.
Robert Gibbs, the White House press secretary, said Thursday that Britain and UAE investigators have the packages.
"The investigators are looking into all features of the investigation," Gibbs said. "Obviously, since those were to be loaded on U.S. carriers, the FBI is deeply involved.
"The forensics are very tedious, meticulous. ... We have confidence in that investigation, and when it's concluded, we'll look forward to that investigation being made public ... but I have no information to confirm that fact."
Investigators in Yemen say they suspect that Ibrahim Hasan al-Asiri, al Qaeda's top bomb maker in the region, is behind the explosive devices sent in the parcels. U.S. authorities are said to be also looking at al-Asiri because PETN was used in the parcels and by the underwear bomber.
CNN's Jeanne Meserve, Jill Dougherty and Becky Brittain contributed to this report.