“Civil rights” violations, naturally. Kind of funny, since Mr. Zimmerman is both a minority (registered as Hispanic for the Census) and a Democrat.
You're right. The problem is DOJ needs to have a legal hook to go after Zimmerman under federal law, which would be either Zimmerman acting under color of law or interfering with Trayvon's civil rights (which would include, for example, using a federal food stamp card to buy skittles but would not include just walking down the street).
So their only angle is that Zimmerman was somehow a state actor acting under color of law.
Some of the Demonrats have been pushing a federal bill to regulate neighborhood watchmen.
Obviously they can't do this under the current view of the Commerce Clause, but they could do this as a civil rights law if they can show a pattern of local neighborhood watchmen violating the civil rights of people.