They didn't take all the servers from the datacenter. Just the ones they had co-located there. Electronic forensics has gotten much more efficient over time. So instead of taking months to return said servers, it should take them on a few days to a few weeks. Unfortunate for those affected.
However, there are many variables on how the servers were configured, how they were managed, how the data was organized, backed up and archived. They take the systems away so they can do anaylysis in a controlled environment.
It does sound like they have a very good idea of what they are looking for. Local news (in dallas) reports today that they are looking for info related to a mass fraud investigation.