The parcel bombs sent from Yemen destined for the United States have hallmarks of the militant group al Qaeda or its affiliate al Qaeda in the Arabian Peninsula, US Homeland Security Secretary Janet Napolitano said on Saturday.
"I think we would agree with that, that it does contain all the hallmarks of al Qaeda and in particular al Qaeda AP," she said in an interview on ABC News.
"The investigation is ongoing and just as we have an adversary who keeps pushing the system, trying to conduct a plot, we are ourselves continuing to make sure we get to the source of this and make sure that we take care of this problem," she said.
The Obama administration has been increasingly focused on the al Qaeda affiliate, which authorities have said was behind the failed attempt to blow up a US jetliner on Christmas Day last year with a bomb that a Nigerian man hid in his underwear.
In a series of television interviews on Saturday, Napolitano also said authorities were checking whether other packages had been sent before the two that were intercepted that could offer clues about attempts to test US security.
"We're doing some reverse engineering as it were to identify other packages from Yemen," she said on NBC News.
But she said the investigation was continuing and that it was too early to say whether the packages were meant to detonate while airborne on the massive cargo planes or when they reached their destinations, Jewish centers in Chicago.
She said on CNN that the parcel bombs appeared to include the same explosives used in the Christmas Day attempted attack, pentaerythritol trinitrate, or PETN.
"We know aviation continues to be a target, we know the system continues to be looked at by our adversaries," Napolitano said.
Air travelers should expect to experience stiffer security measures at US airports, including bomb-sniffing dogs and equipment that can detect trace explosives on bags or passengers, she said.
Napolitano declined to say whether US authorities had detected details of the plot themselves as well or if they relied solely on the information provided by Saudi Arabia, a key US ally.