Apparently in October a National Archives Offical noticed him taking them .. then asked for them back
How the FBI got involved and who notified them, I'm not exactly sure .. but they searched his house and safe in January
Thanks.
Mark Levin for National Review has written an article about why the docs were stolen.
http://www.nationalreview.com/levin/levin200404151634.asp
If I read correctly the archive official went to Berger and asked for the docs back. He did not have them all because he destroyed some so I believe the FBI was then involved in the case.