The EU regulators behind a $5 billion fine against Google are turning their attentions to Amazon.

European Competition Commissioner Margrethe Vestager has begun questioning merchants on Amazon's use of their data, Vestager said Wednesday. The issue, she said, is whether Amazon is using data from the merchants it hosts on its site to secure an advantage in selling products against those same retailers.

"These are very early days and we haven't formally opened a case. We are trying to make sure that we get the full picture," Vestager said during a news conference Wednesday.

The probe comes as the world's largest online retailer faces growing calls for regulation. Investors and insiders have long cited Amazon's size and reach as reason to break the company up. President Donald Trump has hinted at antitrust action against Amazon as part of continued attacks against CEO Jeff Bezos, who also owns The Washington Post.

U.S. Attorney General Jeff Sessions was set to meet this month with state officials to discuss antitrust concerns in Silicon Valley, though much of the regulation on Big Tech thus far has come out of Brussels.

Vestager has the power to fine companies up to 10 percent of their global turnover for breaching EU antitrust rules. Earlier this year, she levied a record $5 billion fine against Google related to its Android business. She also launched an "in-depth investigation" into Apple and its purchase of music recognition app Shazam.

Amazon allows third-parties to sell goods on Amazon through its Seller program. The questioning by Vestager appears to be designed to determine whether or not Amazon puts these third-party sellers at a disadvantage by using their sales data to boost Amazon's own sales.

Amazon declined to comment.

—Reuters contributed to this report.