The L.A. Times says:

The U.S. Justice Department's antitrust arm said it was looking into potentially unfair pricing practices by electronic booksellers, joining European regulators and state attorneys general in a widening probe of large U.S. and international e-book publishers...A Justice Department spokeswoman confirmed that the probe involved the possibility of "anticompetitive practices involving e-book sales."

If this investigation is anything like what's going on in Europe, it centers around Apple's deal with the big publishers to set their own iBooks prices, which has resulted in higher e-book prices everywhere. More information on the European suit can be found at Reuters.