Even if backdoors never made it into the code, I agree with Mouser that if the FBI actually did attempt to get backdoors in place, that's a big deal in itself.
There should be some investigation of whether that allegation is true, though I have no idea how you'd go about doing that. I imagine it would be hard to get anyone with authority and resources interested in pursuing an investigation. So you'd be left with asking the accused, with no way to compel any response (much less an honest one).
I'm not even sure if the alleged FBI behavior is illegal - I wouldn't be surprised if it wasn't. Even if legal, it would still be outrageous, in my opinion.