The US Department of Justice (DoJ) has confirmed that its anti-trust division is investigating Google's $125m (£76m) deal with book publishers made last year.

The deal resolved a number of copyright issues raised by book publishers about the search giant's Book Search service that has digitised millions of works.

The settlement included the founding of a Books Rights Registry that distributes revenue from Google Book Search to authors and publishers.

But Google's backing from US publishers means that the service could become the largest online portal of human knowledge, leading to concerns that it could breach the Sherman Antitrust Act.

The DoJ issued a formal notice of investigation on Thursday, and sent a letter to the US District Court for the Southern District of New York, according to a report in The New York Times.

"At this preliminary stage, the US has reached no conclusions as to the merit of those concerns, or more broadly what impact this settlement may have on competition," said William Cavanaugh, an assistant attorney general for the DoJ.

"However, we have determined that the issues raised by the proposed settlement warrant further inquiry."

Google confirmed in a statement that the DoJ had contacted the company to learn more about the impact of the settlement.

"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," said Google.

Meanwhile, Google has updated its Book Search service by adding search results to users' scrollbars.

"Now when you search in a book, little hints will appear in the margin to indicate where your results are located," said Google software engineer Casey Ho in a blog post.

"When you hover over one of these annotations, you'll get a quick preview of the search results and the option of jumping directly to the associated page."