Homeland Security Secretary Janet Napolitano confirmed Saturday morning that the investigation is ongoing into who is responsible for trying to send explosives to the United States on a cargo plane in a plot thwarted Friday. 

"Well, it's certainly an ongoing investigation," Napolitano said this morning on CNN. "And in addition to the two packages that we had specific information about, we have put in place enhanced protections for cargo and passengers emanating from Yemen and making sure that we identify particularly all packages that are coming from there."

She said federal authorities weren't aware of any other packages left to be found but said the were acting with "an abundance of caution."

Napolitano said authorities haven't been able to determine how the explosive devices, which were apparently directed at two synagogues in Chicago, were going to work. 

"That is an unanswered question," she said. 

A female suspect, alleged to be connected to the plot, was arrested in Yemen today as part of a growing search to find  those responsible for sending two suspicious packages to the United States through the United Arab Emirates and the United Kingdom, according to news reports. 

The suspects were believed linked to al-Qaeda in the Arabian Peninsula, the group's affiliate in the Persian Gulf, Yemeni officials said today

Preliminary tests showed that that the explosive material was PETN, a substance used by that al-Qaeda affiliate in other planned attacks, including the attempt to bring down a commercial airliner over Detroit on Christmas. 

Authorities scrambled to check planes in the U.S., England and the United Arab Emirates (UAE).

Bombs found in Dubai and Britain were active and could have exploded at any time. Discovered in a FedEx facility, the Dubai package contained white powder explosives discovered in the ink cartridge of a printer. The device contained an electric circuit and a cellphone chip.

Although U.S. officials have said the mail bombs found in the UAE and England were headed to Chicago, the bombs may also have been intended to blow up the cargo plane itself, British Home Secretary Theresa May said today. 

"We believe that the device was designed to go off on the airplane," said Prime Minister David Cameron. "We cannot be sure about the timing when that was meant to take place.There is no early evidence that that was meant to take place over British soil, but of course we cannot rule it out."