No one's absolutely certain, but the following article makes a pretty good case for Crypto AG:

https://web.archive.org/web/20080101082631/http://www.inteld...

Edited to add:

I tried chasing down a few of the links mentioned in the comments of https://www.schneier.com/blog/archives/2008/01/nsa_backdoors..., but I hit some dead ends and lack time to give you further detail. Sorry about that. As far as I can tell, the original report comes from Der Spiegel.