Google said it was happy to answer the Department of Justice's questions
US anti-trust regulators are to investigate a $125m (£76.4m) deal Google has made with book publishers to settle copyright issues, reports say.
The settlement compensates copyright holders and gives Google a share of online book sales and advertisements.
The deal "warrants further inquiry", US Deputy Assistant Attorney General William Cavanaugh said in a letter filed to the New York District Court.
Some fear the deal could make Google the main source for many online books.
"The US has reviewed public comments expressing concern that aspects of the settlement agreement may violate the Sherman [Anti-Trust] Act," Mr Cavanaugh said.
"At this preliminary stage, the US has reached no conclusions as to the merits of those concerns or more broadly what impact the settlement may have on competition," he added.
Right to read
In October 2008, Google reached a deal with the Authors Guild and the Association of American Publishers.
The search engine agreed to pay $125m to create a Book Rights Registry, where authors and publishers can register works and receive compensation.
Google can also digitise orphan works - works whose rights-holders are unknown. Some fear the settlement could prevent other companies from entering the digital book market.
"The Department of Justice and several state attorneys general have contacted us to learn more about the impact of the settlement, and we are happy to answer their questions," Google said in a statement.
"It is important to note that this agreement is non-exclusive and if approved by the court stands to expand access to millions of books in the US."
Jeff Bezos, chief executive of Amazon.com, has previously expressed concerns about the settlement and has said he believes "it needs to be revisited".