The European Union is taking steps that could lead to a third antitrust complaint against Google, this time over its lucrative advertising services, according to three people familiar with the EU investigation.

EU officials sent the search giant’s critics requests to allow their evidence to be shared with Google, said the people, who asked not to be named because the case is private.

One request was sent as recently as last week, one person said. Such a move is typically a precursor to a formal EU statement of objections cataloging how a firm may have violated EU antitrust law.

The investigation targeting AdWords could affect Google’s revenue stream more than either of the other two probes. The targeted ad service, which displays promotions alongside query results and web content, has been a key driver in helping Alphabet’s Google grow its sales. Most of Google’s US$74,5bn 2015 revenue came from advertising.

First announced more than five years ago, the probe has targeted contracts with websites that shut out non-Google advertising services and deals with computer and software vendors that prevent them from using other search tools.

The European Commission declined to comment. Google didn’t immediately respond to a request for comment. Politico reported on the requests earlier on Monday.

Margrethe Vestager, the EU’s competition commissioner, said last month that she hoped the investigation into Google’s AdWords advertising service could conclude “within a reasonable timeframe”.

Google is already fighting two other investigations that the EU has escalated since Vestager took over as EU antitrust chief in late 2014. It was charged last year with unfairly promoting its own services over rivals in shopping search services. In April, it got a formal complaint over its Android mobile operating system that it must still answer by the end of the summer.