Tong released only a few details of the probe. He said the state attorney general's office has previous taken action against Apple and e-book publishers to protect competition in the marketplace.
“Our office continues to aggressively monitor this market to protect fair competition for consumers, authors, and other e-book retailers,” Tong said in a statement.
An Amazon spokesperson declined to comment Thursday.
The investigation is part of the widening scrutiny by state and federal government agencies of possible anticompetitive practices by giant tech companies. Google and Facebook are facing similar probes by federal and local officials into whether their business practices are illegally squashing competition and harming consumers.
Amazon has become the dominant force in print book sales and e-book sales in the U.S. The company accounts for over half of all print book sales and more than 80% of e-book sales, according to research cited in an October report by the antitrust committee of the U.S. House Judiciary Committee.
Tong's office issued a subpoena to Amazon in 2019 requesting documents about the company's dealings with five book publishers: HarperCollins Publishers, Hachette Book Group, Penguin Random House, Simon & Schuster and Macmillan. A copy of the subpoena was obtained by the nonprofit investigative Tech Transparency Project and shared with The Wall Street Journal, which reported on the investigation Wednesday.
Simon & Schuster declined to comment. Messages seeking comment were left with the other four publishers Thursday.
In a previous antitrust investigation of electronic books, Apple was found to have conspired with publishers to raise e-book prices in an effort to challenge Amazon's dominance of the market. Apple fought the findings all the way to the U.S. Supreme Court, which rejected the appeal in 2016 and let stand a lower court ruling that found Apple violated antitrust laws in 2010.
The Justice Department and 33 states and territories originally sued Apple and five publishers. The publishers all settled and signed consent decrees prohibiting them from restricting e-book retailers’ ability to set prices.
In settlements of lawsuits brought by individual states, Apple agreed to pay $400 million to be distributed to consumers and $50 million for attorney fees and payments to states.