WASHINGTON — Makan Delrahim, the head of the antitrust division at the Department of Justice, has recused himself from investigating Google, even as the agency’s examination of the largest tech companies ramps up.

Mr. Delrahim, 50, recently removed himself from looking into allegations of anticompetitive practices at Google because of a potential conflict of interest related to his past work for the internet search company, two people with knowledge of the decision said.

In 2007, Mr. Delrahim, who was in private law practice at the time, had a contract to lobby for Google’s acquisition of the ad-technology company DoubleClick, according to the people, who spoke on the condition of anonymity because the details are confidential.

“As the technology review progressed, Assistant Attorney General Makan Delrahim revisited potential conflicts with previous work with the Department of Justice’s ethics office,” a Justice Department spokesman said. “He and the ethics office have decided that he should now recuse himself from a matter within the tech review in an abundance of caution.”