The US Attorney's Office (DOJ) can bring a civil suit , or it can seek a criminal indictment (usually by way of a grand jury).
The primary differences are these.
1. A criminal case are offenses against the state (government). remember victims (people) never 'press charges'. The state or the government does this, sometimes against the victim's wishes (which may end up torpedoing the State's case).
2.. Burden of proof. Civil actions have a lower burden (preponderance of the evidence), Criminal has a higher burden (beyond a reasonable doubt).
3. Punishment: Only Criminal matters could result in the loss of a person's freedom, not to mention fines. Civil, is either monetary damages, or a party stopped from doing something or forced to do something.
4. Criminal: right to trial by jury. Civil, might have a jury trial, likely in front of a judge.
5. Criminal: defendant has right to counsel, Civil: no such right exists.
6. Defendants have much more constitutional protection in a criminal matter vs a civil matter.
The #1 reason this is a federal case is the nature of the alleged crime (wire fraud, and money laundering), and how/where the informants surfaced. Could the US bring a civil suit against these people? I suppose they could, I haven't really thought about that angle, and what their cause of action would be.