errr - how much of that linked open letter is true. It states that NSA insiders had evidence that 9/11 attackers were monitored by NSA, calling AlQueda safe houses from the US to Yemen and shared nothing about it. and this was buried / covered up.
The EFF has been supporting these former NSA agents' whistleblowing efforts for a while now. Also, Ellsberg - author of the Pentagon Papers - has signed it in support.
While it's not a character reference, it is worth observing that the rather dramatic efforts the NSA has gone though to threaten and prosecute these whistleblowers. You may be interested in watching this talk from 29c3 where Drake and Binney told their stories in their own words:
It's also the one that finally made this NSA mess make sense, with their description of THINTHREAD being dropped because it was too inexpensive. It's all about moving money to their contractor friends.
I hadn't heard of this letter before the parent comment's mention of it, but the names mentioned are NSA whistleblowers who might have had the knowledge and access to substantiate whatever claims they make.
that's a hell of an accusation