The Department of Justice's top antitrust official has recused himself from the Department's investigation into whether Alphabet's (GOOGL  ) Google has participated in unlawfully suppressing the competition.

The department said that as the probe progressed, Assistant Attorney General Makan Delrahim realized that he needed to recuse himself due to his past work with the company. In 2007, Delrahim advised Google as it sought to approval from the Federal Trade Commission to buy internet ad firm Double Click. Double Click helps strengthen Google's advertising business, which has been a major focus for the antitrust investigation.

Mr. Delrahim "revisited potential conflicts with previous work with the Department of Justice's ethics office. 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," a department spokesman said.

The department opened investigations into Google, Amazon (AMZN  ), and Facebook (FB  ) last summer as questions over anti competitive practices increased. The Justice Department has since called in dozens of rivals across media, retail, and tech to gather evidence of noncompetitive practices by the tech giants. The reviews are expected to place for most of 2020.

Critics including Democratic presidential candidate Sen. Elizabeth Warren, have been called for Delrahim's recusal due to his past history. Warren said in a June letter to Delrahim that he should recuse himself from any probes of Apple because of his role in their 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 in the letter. "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."

It is unclear why Delrahim recused himself from the investigation now given the length of the Department's investigation into Google's practices.