They could be 'janitorial engineers' for all we know. The term has become pretty loose especially with the lack of actual education in universities now.
As far as terrorists go, I think we can all agree that if they are engineers, they've been FIRED from their other job. I mean the last few guys that I heard about built some improvised explosive, were in position and ready to detonate, and the bomb DIDN'T EVEN WORK. Even I could do better than that, and I'm just a software engineer.
Terrorist, or just a regular idiot? You make the call.