According to an interview with Politico, Cheney says it's number 1 or 2 between nuclear bomb and biological attack on a major American city with DC being number one on that list. I still have to assume he probably has more insight and knowledge then us.

His quote from the interview.

“the ultimate threat to the country” is “a 9/11-type event where the terrorists are armed with something much more dangerous than an airline ticket and a box cutter – a nuclear weapon or a biological agent of some kind” that is deployed in the middle of an American city.