The EU has launched an investigation into whether Amazon is violating competition law.

The news: The EU is conducting a preliminary inquiry into how Amazon uses data from sellers in its online marketplace. It will look at whether Amazon’s dual role as both merchant and platform is anti-competitive—for example, whether the company could be using data from sellers to undercut them. Amazon did not comment.

Background: European Union commissioner Margrethe Vestager has a fearsome reputation in Europe. She fined Google $5 billion in July and in 2016 ruled Apple must hand over $16 billion in back taxes to the Irish government, which it finally paid this week.

Is this needed? Amazon will account for half of all online sales in the US by the end of 2018. Only time will tell whether it can be deemed to have broken competition rules, but its sheer size means it’s attracting growing attention from antitrust authorities.

Could the US follow suit? The US and the EU interpret competition differently. The US tends to focus on whether the company’s behavior is pushing up prices for consumers. The EU has a broader remit to investigate companies deemed to have a “dominant position” in a market.