“Finally, Sedensky asserted that disclosure of the affidavits would “seriously jeopardize the outcome and success of the investigation by divulging sensitive and confidential information known only to investigators and any suspect(s) and also identify persons cooperating with the investigation, thus possibly jeopardizing their personal safety and well-being,” the applications show.”
http://www.countytimes.com/articles/2012/12/29/news/doc50def85374685859116293.txt
Threat of secret prosecution can help keep those that know in line.