The top antitrust regulator in the European Union said Wednesday she is investigating Amazon's relationship with independent merchants it hosts on its platform.

Why it matters: If this were to turn into a formal investigation, it would represent a major headache for Amazon, as it faces allegations that it has unfair dominance over the market.

What they're saying: EU competition Commissioner Margarethe Vestager pointed to the possibility that Amazon could use data on independent merchants who use its platform to refine its own offerings as a seller itself. She said that "the question here is about the data."

• Vestager said it was "very early days" and that the authorities have not "formally opened the case."
• Amazon declined to comment on the record.

The big picture: Debates over Amazon's dominance have fueled a resurgent antitrust movement in the United States, but European regulators have actually taken action against the tech giants.