The US Department of Justice (DOJ) has become more aggressive in its antitrust probe of Alphabet Inc.'s Google, according to a Wednesday report form The Wall Street Journal.

Investigators have "been posing increasingly detailed questions" to Google rivals and company executives, the WSJ story says, including on the topic of Google's third-party advertising business.

Two focuses of the investigation are the company's integration of its ad server with its ad exchange and the company's policy that YouTube advertisers must use Google solutions to buy ads on the platform.

The DOJ has contacted "more than a dozen companies" across different industries as it proceeds with its investigation, according to the report.

Google didn't immediately return a request for comment.