The European Commission found potential competition concerns in a report on smart devices and virtual assistants that named Apple, Amazon and Google as the leading companies in the sector.
The preliminary findings, released on Wednesday, "could lead to new competition cases being opened in the future," said Margrethe Vestager, the commission's executive vice president, in a statement.
The commission and countries in the bloc have often been at the forefront of antitrust enforcement in a variety of digital markets.
Vestager cited "concerns about attempts to restrict the number of voice assistants accessible on smart devices" and limits by producers on installing second voice assistants in addition to Apple's Siri, Amazon's Alexa and others.
She also mentioned the possibility that voice assistant providers "promote their own services or the services of third-parties of their choice" and worried over the amount of data they collect.