Alphabet unit Google on Tuesday found itself in the EU antitrust spotlight again as regulators opened an investigation into its lucrative digital advertising business to examine whether it favors its own business over rivals, advertisers and online publishers.

Google generated $147 billion in revenue from online ads last year, more than any other company in the world. Ads on its properties, including search, YouTube and Gmail, accounted for the bulk of sales and profits.

About 16% of revenue came from its display or network business, in which other media companies use Google technology to sell ads on their website and apps.

The European Commission said it would investigate whether Google distorts competition by restricting access to third parties to user data for advertising purposes on websites and apps, while reserving such data for its own use.

“We are concerned that Google has made it harder for rival online advertising services to compete in the so-called ad tech stack,” European Competition Commissioner Margrethe Vestager said in a statement.

“We will also be looking at Google’s policies on user tracking to make sure they are in line with fair competition,” she said.

Google said it would engage constructively with the Commission.

“Thousands of European businesses use our advertising products to reach new customers and fund their websites every single day. They choose them because they’re competitive and effective,” a spokesperson said.

Reuters reported exclusively last week that the EU competition enforcer would investigate lucrative digital advertising business before the end of the year.

Vestager has in the last decade fined the company more than 8 billion euros ($9.5 billion) for blocking rivals in online shopping, Android smartphones and online advertising.