LONDON — European Union regulators say they’re opening an in-depth investigation into U.S. tech giant Google’s plan to buy fitness tracking device maker Fitbit

The EU’s executive commission said today it’s concerned the deal would entrench Google’s position in the online ad market by “increasing the already vast amount of data” that the company could use to personalize ads.

EU competition commissioner Margrethe Vestager said the “investigation aims to ensure that control by Google over data collected through wearable devices as a result of the transaction does not distort competition.”

Google agreed to buy Fitbit in November for $2.1 billion. Privacy and consumer groups have called on authorities to block the deal, citing privacy and antitrust concerns.