The music download industry is remaining tight lipped as the US Department of Justice launches a probe into allegations over price fixing for downloads.

The DoJ has confirmed that four music companies (Sony BMG, Warner Music Group, Universal Music Group and EMI) have been targeted by the probe.

It is believed that subpoenas have been issued to all four labels over allegations that they colluded to fix prices for music downloads.

"As part of an industry-wide DoJ inquiry, we received a request for information related to pricing of digitally downloaded music," a Warner Music Group spokeswoman told said.

"As is the case with the previously disclosed industry-wide request from the New York Attorney General on this subject, we intend to cooperate fully."

An EMI spokesman confirmed that the company would not be making any comment due to the ongoing nature of the case. EMI saw its digital music sales climb by over 140 per cent last year.

Napster refused to comment on the investigation as did Apple, which runs the most popular download service, iTunes.

Apple chief executive Steve Jobs has previously described the attitude of music companies as "greedy", although iTunes has come under fire for charging UK consumers excessively.

Some commentators have suggested that the music companies may have a real fight on their hands. The industry has already been fined for CD price fixing in the US, and the European Union claims to have found evidence of similar sharp practice.