NoHeat
En-Route
If you read the article, red teams are able to get explosives past screening 95% of the time. And at least two real terrorists have smuggled bombs past TSA screening and onto planes. That's not even counting the dozens or hundreds of people who've accidentally gotten past screening with prohibited weapons or explosives. So to whatever extent "something is working," we can say for sure that it isn't the screening.
Underwear Bomber: http://en.wikipedia.org/wiki/Umar_Farouk_Abdulmutallab
Shoe Bomber: http://en.wikipedia.org/wiki/Richard_Reid
The screening even missed the C4 in this guy's bag after an x-ray and hand search: http://usatoday30.usatoday.com/news/story/2012-01-03/soldier-airport-explosives/52360742/1
Yes, it is clear that explosives are the biggest threat now, in the air. Armored cockpit doors have pretty much eliminated the threat of hijacking.
I'd bet that compared to a knife, a small brick of plastic explosive is harder to spot in an X-ray. Regardless of who employs the X-ray worker, it's going to be hard for them to do that job well enough.