It's old news by now that, as CNN recounts:

After a meeting with state attorneys general and federal prosecutors from Louisiana, Alabama and Mississippi, U.S. Attorney General Eric Holder told reporters that the Justice Department was looking at possible criminal violations in connection with the spill.

When I was in the Justice Department  --  under administrations of both parties  --  we waited until the end of the investigation to go public with it.  We did not announce an investigation at the beginning, lest innocent parties be unfairly tarred, and lest guilty parties make haste to hide or destroy evidence.  Evidently things have changed.

Why?

Because the administration is in big political trouble on account of the oil spill, see Real Clear Politics story here, and has to pretend it's doing something.  The public knows full well that it has massively failed to do the one thing that actually needs doing, i.e., capping the pipe.  Given Big Government's now proven inability to do that, it seems that Plan B is to have the Attorney General use his official position to snarl at the boogeyman, that being BP.

BP makes a great boogeyman, for sure.  It may be guilty of all kinds of crime, and one way or the other, the results of its behavior are a catastrophe.  But, as Mr. Holder knows full well  --  for example, in dealing with terrorist detainees  --  our system prizes the presumption of innocence.  Merely because it's convenient as a political matter, and necessary as a PR matter, to forget that in this instance does not make it any less a part of the system of law Mr. Holder elsewhere claims to value.