> Honestly, if a three letter agency was working with a tech company to produce a back door, the last people I would expect to know would be most of the engineers involved in the implementation.

Who would the first people be then?

If I were a 3 letter agency with a large budget, I would insert someone as a project manager at the target company. If I couldn't do that, I would work with their C level folks.

Preferably no one in the implementing company. Work using customer pressure, say an important bank. And later you swoop in and get exclusive access during a nice and cozy dinner with one of the Cs. It's more like judo than brute force arm twisting.