I saw a while back that this was happening, it may just be making the news now, but it has been underway since shortly after the new Mayor took office and it is at his request.
He came into office with three unrelated murder investigations underway against officers of what is reputed to be the most corrupt and abusive police force in the country and determined to take action so he requested a DOJ investigation.
The fact that it is publicized now is purely political the actual investigation started a couple years ago.