The Justice Department has reached out to more than a dozen companies in its antitrust probe of Google, including publishers, advertising technology firms and advertising agencies, as the company’s online ad tools become a major focus of the investigation, according to people familiar with the matter.

In recent months, the department has been posing increasingly detailed questions—to Google’s rivals and executives inside the company itself—about how Google’s third-party advertising business interacts with publishers and advertisers, the people said. That digital business was built largely on the company’s 2008 acquisition of the ad-technology firm DoubleClick.

Many of the questions center around two moves that Google has made in recent years that publishers and rivals say have helped consolidate its power. The first was Google’s integration of its ad server, the leading tool for websites to put ad space up for sale, with its ad exchange, the industry’s largest digital ad marketplace. The second move was its decision to require advertisers to use its own tools to buy ad space on YouTube.