Makan Delrahim, U.S. assistant attorney general for the antitrust division, listens during a Bloomberg Television interview at the Milken Institute Global Conference in Beverly Hills, California, U.S., on Tuesday, April 30, 2019. The conference brings together leaders in business, government, technology, philanthropy, academia, and the media to discuss actionable and collaborative solutions to some of the most important questions of our time. Photographer: Patrick T. Fallon/Bloomberg via Getty Images

Makan Delrahim, the top antitrust official at the Department of Justice, is recusing himself from the agency's investigation of potentially anticompetitive behavior at Google due to potential conflicts of interest in his past.

Delrahim was previously hired as a lobbyist who helped shepherd Google's $3.1 billion acquisition of DoubleClick in 2007. DoubleClick has become a key part of Google's advertising business, which has raised antitrust concerns.

"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 spokesperson said in a statement to CNBC after The New York Times first reported the recusal. "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. Associate Deputy Attorney General Ryan Shores will continue to oversee the tech review. Deputy Assistant Attorney General Alex Okuliar, who joined the Antitrust Division last week, will also assist with the tech review."

Shores previously worked as in the litigation and antitrust department at law firm Shearman & Sterling. He was hired in October by Deputy Attorney General, Jeffrey Rosen, to work in his office, which oversees all of the Justice department divisions, rather than Delrahim's antitrust division. The Justice Department said at the time he would work closely with its antitrust review, so he has been overseeing the investigation since then.

The change of leadership overseeing the Google review may further the investigation's ties to Deputy Attorney General, Jeffrey Rosen's boss, Attorney General William Barr, industry sources noted. Barr has appeared to cooperate in working with states that are leading their own antitrust investigation into Google, a person familiar with the investigation said, requesting anonymity because the investigations are confidential.

The timing of the decision, months after the DOJ announced it was an investigation, took some industry insiders by surprise. It could suggest that the DOJ is zeroing in on Google's acquisition of DoubleClick as part of its review, industry sources noted. DoubleClick helps strengthen Google's advertising technology business, which has been a major focus for a separate antitrust investigation attorneys general are leading.

The DOJ and the states have been in touch about their respective investigations, people familiar with the situation said.

Critics including Sen. Elizabeth Warren, D-Mass., have called for Delrahim's recusal in light of the potential conflict. Warren also said in a June letter to Delrahim that he should recuse himself from any probes of Apple since he had previously been hired to lobby federal officials on its behalf on patent reform.

"As the head of the antitrust division at the DOJ, you should not be supervising investigations into former clients who paid you tens of thousands of dollars to lobby the federal government," Warren wrote at the time. "American consumers and markets deserve the confidence that the DOJ will conduct any antitrust investigation into Google or Apple with integrity, impartiality, and with the best interest of competitive markets and consumers in mind."

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