US antitrust enforcers have stepped up an investigation into Google's settlement with publishers over its book-scanning project.
The US Justice Department has issued formal requests for information to several of the parties involved, including Google, according to the New York Times.
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Google reached an agreement in October 2008 to create a $125m fund to pay authors to have their work scanned and made available online.
The US Justice Department became involved after representatives for Consumer Watchdog and the American Antitrust Institute raised concerns earlier this year.
Lawyers who represented the Authors Guild in negotiations with Google said the information requests indicates US authorities are serious about the antitrust implications of the settlement.
The settlement was aimed at resolving a class action by the Authors Guild and the Association of American Publishers against Google.
Under the settlement, Google would have the right to display the books online and to profit selling access to them, but revenue would be shared with authors and publishers.
The settlement still needs federal court approval and the deadline has been extended to September for other authors and publishers to oppose it.
Google has defended its position by highlighting the benefits to readers of putting millions of books out-of-print books online.
The Justice Department's latest requests do not necessarily mean that the US government will oppose the settlement, but the investigation could delay approval of the settlement, analysts said.