The attorney general for the State of Connecticut has launched an investigation into two of the largest sellers of e-book titles.
Richard Blumenthal said that his office will undertake an investigation of Apple and Amazon over possible anti-trust violations.
Specifically, Blumenthal intends to find out whether the companies use their size in the marketplace to prevent other retailers offering cheaper titles.
The investigation comes as Apple and Amazon report booming sales for their respective devices. Apple has already sold more than three million iPads, while Amazon recently announced that its Kindle store now sells more e-books than printed books.
"Amazon and Apple combined will likely command the greatest share of the retail e-book market, allowing their most-favoured-nation clauses to effectively set the floor prices for the most popular e-books," said Blumenthal.
"Such agreements, especially when offered to two of the largest e-book retail competitors in the US, threaten to encourage co-ordinated pricing and discourage discounting."
Blumenthal takes a tough line with technology companies, and was among those who threatened to take Google to court over the Street View Wi-Fi snooping scandal.
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