The US Department of Justice (DoJ) is to investigate the price of eBooks, just days after the EU announced a similar anti-trust probe.

"We are also investigating the electronic book industry, along with the European Commission and the states attorneys general," Sharis Pozen, acting antitrust chief at the Justice Department, said at a congressional hearing, according to the Wall Street Journal.

Unlike the EU, which confirmed this week it will investigate Apple and five publishers including Penguin and Harper Collins, the DoJ did not reveal the names of the firms that will be subject the investigation. The DoJ's announcement marks the first time the regulator has confirmed it is looking at eBook pricing.

The EU has questioned whether the firms have "engaged in anti-competitive practices" regarding the price of eBooks, "possibly with the help of Apple", which breaks EU antitrust regulations. In particular, the commission is concerned competition has been hampered after the five publishers adopted the 'agency model' which saw them setting the price for eBooks sold through Apple's store, iBook, with the revenue split between the publisher and Apple.

"The Commission will in particular investigate whether these publishing groups and Apple have engaged in illegal agreements or practices that would have the object or the effect of restricting competition in the EU or in the EEA," the EU said this week.

The EU also confirmed that the UK Office of Fair Trading (OFT) previously carried out a similar investigation, which began in February this year. Despite the investigation being closed "on grounds of administrative priority" the OFT will "co-operate closely with the Commission going forward".