Connecticut Attorney General Richard Blumenthal has written to Apple and Amazon asking them to address concerns over their e-book deals with publishers.
He claims that a preliminary review has found identical prices for several bestsellers on both companies' stores, as well as those of Borders and Barnes & Noble.
"These agreements among publishers, Amazon and Apple appear to have already resulted in uniform prices for many of the most popular e-books - potentially depriving consumers of competitive prices," Blumenthal said in a statement.
"Such agreements - especially when offered to two of the largest e-book retail competitors in the United States - threaten to encourage coordinated pricing and discourage discounting."
Blumenthal's comments come as Amazon claims to hold up to 80 per cent of the e-book market.
Get the full story on the probe into the two firms over at Mobile Entertainment.