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"It's hard to imagine what we could have done differently or more quickly," says Homeland Security Secretary Janet Napolitano of last week's discovery of a plot to send bombs to the U.S. by cargo plane.
News that a plot to send bombs to the United States was foiled by an informant — and not by inspections — should not be taken as a sign that the security system didn't work, says Department of Homeland Security Secretary Janet Napolitano.
"We use a multilayered system, out of which intelligence-sharing is the first layer," she told NPR's Robert Siegel in an interview Monday afternoon for All Things Considered.
And since the terrorist attacks of Sept. 11, 2001, Napolitano said, those intelligence-sharing arrangements are becoming "evermore robust."
Still, it's been reported that it took 20 hours for British authorities to determine that the parcel in question contained explosives. That suggests that without advance warning, it might not have been detected.
Asked if a standard airport inspection would have detected the explosives in the packages, Napolitano said, "The plain fact of the matter is, we did — and that wasn't by accident."
Still, law enforcement agencies have not yet ruled out the possibility that there may be other bombs out there, similar to the two bombs that were detected late Thursday and early Friday.
"We've put a ground halt on all cargo emanating out of Yemen, until they can be inspected," Napolitano said.
Asked how the bombs, which were hidden in ink toner cartridges in printers, might have been detonated, Napolitano said, "The forensics investigation is not yet complete," but that officials have a "general idea" of how the devices were meant to be detonated. She added that she did not want to discuss the detonation details publicly.
The terrorists' plot to use commercial cargo carriers like UPS was a new tactic— part of what Napolitano says is a constantly changing threat.
In that environment, the United States can't afford to be be predictable, the security head said. "We're constantly changing, as well," she said.
The explosives in the toner cartridges were familiar to investigators, Napolitano said — the compound has been identified as PETN, a nitroglycerin explosive that was also used in a plot last Christmas which also had ties to Yemen.
Now investigators will "reverse-engineer" the packages, she said, to see how they were made — and how they were intended to be used.
The U.S. cargo system has long been seen as vulnerable to an attack, primarily because of its size and expanse.
"You've got millions and millions of pounds of cargo that moves around the globe via cargo plane," Napolitano said.
That means it would be extremely difficult to inspect every shipment — something proposed by Rep. Ed Markey of Massachusetts, who wrote the 2007 law for screening cargo on passenger planes.
Napolitano says that screening 100 percent of the cargo traffic is not yet possible. But, she said, by accounting for risk and other factors, the security system is effective. She pointed to last week's interception as proof.
As for Anwar al-Awlaki — the American radical imam in Yemen, whom many consider to be the man behind the cargo bomb plot — Napolitano said that the attempted attack has the "hallmarks" of his activities.