Koskinen said her emails were on tape but it was too complicated to retrieve them, and they didn’t bother tying to retrieve them - at least not to his knowledge.
Jazon Chaffetz starts at about 1:50:00.
http://www.c-span.org/video/?320097-1/irs-targeting-investigation
The tapes are stored offsite which means they'd have to be signed out by someone. That makes retrieval followed by "destruction" or "accidental loss" complicated. They need to find a Sandy Berger to make the retrieval. ;-)