Airport screeners missed bomb parts

So goes the headline of a lead Yahoo news story. The story continues:

WASHINGTON - Government investigators smuggled liquid explosives and detonators past airport security, exposing a dangerous hole in the nation's ability to keep these forbidden items off of airplanes, according to a report made public Wednesday.

http://news.yahoo.com/s/ap/20071115/ap_on_go_ca_st_pe/airport_security

If this is the case, that is, government investigators are able to smuggle explosives past airport security, why aren't real terrorists getting aboard planes and committing acts of terrorism? If it's that easy to get past Transportation Safty Administration security, then why aren't all those terrorists that are supposed to be plotting to blow-up or hijack planes doing just that - smuggling bombs aboard planes and blowing them up or hijacking them?

The reason terrorist aren't doing just that is obvious: there are no terrorists within the United States who are trying to smuggle bombs aboard US flights.