WASHINGTON—The U.S. Justice Department on Wednesday confirmed that it is investigating the pricing of electronic books, the latest antitrust watchdog to probe whether there was improper collusion by Apple Inc. and publishers to prevent discounting.

At a congressional hearing, Sharis Pozen, the Justice Department's acting antitrust chief, said: "We are also investigating the electronic book industry, along with the European Commission and the states attorneys general."

The...