The publishers under scrutiny in the US are reportedly the same as those investigated in Europe: Hachette Livre, Harper Collins, Simon & Schuster, Penguin and Verlagsgruppe Georg von Holzbrinck,
The basis for the regulatory concern is the replacement of the traditional wholesale model (where retailers purchase products from publishers and then sell them at whatever price they see fit, even if that meant making a loss) with an agency model (where the publishers set the retail price and the retailer - in this case Apple - takes a commission).
This had the effect of pushing up ebook prices, in some cases above that of the print edition at low-price retailers.
There are suggestion that some of the companies are already trying to avoid a court case by reaching a settlement with the Department of Justice. It remains to be seen whether the others will follow suit, or if they are sufficiently confident of their position to risk the expense, negative publicity and potentially worse outcome of a trial.