Amazon is the latest US tech firm to face the scrutiny of regulators in Europe.
The European Commission has opened a new investigation into the retail giant, this time putting its ebook business under the microscope.
Brussels will be taking a closer look at Amazon's contracts with publishers and whether the deals are anticompetitive and make it more difficult for other ebook distributors to compete, violating the EU's antitrust rules.
"Our investigation does not call that [Amazon's business] into question," said the new competition tsar Margrethe Vestager.
"However, it is my duty to make sure that Amazon's arrangements with publishers are not harmful to consumers, by preventing other e-book distributors from innovating and competing effectively with Amazon. Our investigation will show if such concerns are justified."
Vestager has come down hard on companies ploughing ahead with a swell of anti-trust probes since taking up the mantle from Joaquin Almunia late last year.
Amazon and fellow Silicon Valley stars such as Apple, Google and Facebook are already facing questions about tax arrangements in the region. Google is also under the watchful eye of Brussels over its search dominance.
In the latest probe, the Commission said it will "in particular investigate certain clauses included in Amazon's contracts with publishers".
"These clauses require publishers to inform Amazon about more favourable or alternative terms offered to Amazon’s competitors and/or offer Amazon similar terms and conditions than to its competitors, or through other means ensure that Amazon is offered terms at least as good as those for its competitors."
Amazon's ebook deals had faced pushback from the world's biggest publisher HarperCollins and Hachette over the terms, escalating into very public rows.
The tech company now faces further scrutiny over the ebook deals, despite eventually settling the rows with publishers.