Washington (CNN) -- The United States is taking additional undisclosed security steps in the wake of explosive devices found in two air cargo packages, but is not yet following Britain's lead in halting unaccompanied air freight from Somalia, Homeland Security Secretary Janet Napolitano said Monday.
In an interview with CNN, Napolitano said the two devices found on air freight planes headed to the United States bore "all the hallmarks" of a plot by al Qaeda in the Arabian Peninsula, the Yemen-based arm of the terrorist network.
However, she said forensic testing continued on the detonators of the two explosive devices to try to determine if they were intended to go off while the planes were in the air or after they landed in the United States.
The two packages were addressed to locations in Chicago, Illinois, associated with Jewish synagogues.
Napolitano said the United States was deploying additional security measures, but added they were "not measures that we are making public because that would defeat the actual purpose."
Asked about Britain's decision to stop receiving air freight from Somalia, Napolitano said the United States was not taking that step at this time.
Both countries have halted air freight from Yemen.
CNN's Jeanne Meserve contributed to this report.