Well, we don't know what kind of ordnance was used, yet. Once the authorities release that information, we can speculate more. However, I wouldn't think a very bright engineering student from a very bright background would find it difficult to create an explosive. The terrorists are surely the most barbaric minds on earth, but they manage to create these things.
That's true, but generally an engineer goes through design and testing phases before finishing a project with a working prototype. We need some evidence that this kid did the design and testing work, and some evidence of how he obtained the explosives. He may not have done a full explosion test--maybe he skipped that step. But they need to find some design drawings that he wrote and find out where he got the explosives. That was some powerful stuff he used and not something he could buy at the local gun shop.