The Justice Department probe of the Google Books settlement is heating up. On Thursday afternoon, the agency officially opened an investigation into the deal, which would allow the search sovereign to make millions of books available online.

“The United States has reviewed public comments expressing concern that aspects of the settlement agreement may violate the Sherman Act,” wrote William F. Cavanaugh, a deputy assistant attorney general. “At this preliminary stage, the United States has reached no conclusions as to the merit of those concerns or more broadly what impact this settlement may have on competition. However, we have determined that the issues raised by the proposed settlement warrant further inquiry.”

The move is the strongest sign yet that the DOJ may block the settlement, which critics claim would grant Google (GOOG) a monopoly on orphaned works–copyrighted texts without an identifiable copyright holder. The notification, included below, is the first time the DOJ has confirmed the investigation publicly and said that it is indeed looking at possible violations of the Sherman Antitrust Act.

Odd that the letter was filed this week inasmuch as the “fairness hearing” that will determine whether final approval is given to the settlement is still months away. Clearly, Google’s recent public relations offensive claiming the company is just as vulnerable to competition as anyone else hasn’t had much effect.

Reached for comment, Google spokesperson Gabriel Stricker offered the following statement:

“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.

It’s 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 U.S.”

2009.07.02 Order _ DOJ Letter –