Two top Republican senators are pressing the Department of Justice’s inspector general over its investigation of an FBI agent who worked on Special Counsel Robert Mueller’s Russia investigation as well as on the Hillary Clinton email probe.

Wisconsin Sen. Ron Johnson and Iowa Sen. Chuck Grassley sent a letter on Wednesday to Michael Horowitz, the DOJ watchdog, seeking information about its investigation of Peter Strzok.

Strzok was kicked off of Mueller’s team over the summer after the inspector general discovered that he had exchanged anti-Trump and pro-Clinton text messages with Lisa Page, an FBI lawyer with whom he was having an affair.

Page worked on the Mueller team through July.

The circumstances of Strzok’s demotion — he now works in the FBI’s human resources division — remained a mystery for nearly four months and came to light through leaks reported on over the weekend.

Strzok’s political leanings are especially significant because of his central role in both the investigation into Russian interference in the presidential campaign and the Hillary Clinton email probe.

As the FBI’s No. 2 counterintelligence official, Strzok helped open both the Clinton and Russia investigations. He was tapped at the end of July 2016 to lead the Russia probe, which involved an investigation into possible collusion between the Trump campaign and Kremlin.

Strzok conducted many of the biggest interviews in both of the cases, though the outcomes have been starkly different.

He helped interview Michael Flynn on Jan. 24. The former national security adviser pleaded guilty last Friday to lying during that interview about contacts he had last December with Russia’s ambassador.

During the Clinton investigation, Strzok interviewed Clinton about her use of a private email server that to house classified documents. He also interviewed top Clinton aides Huma Abedin, Cheryl Mills, Heather Samuelson and Jake Sullivan.

As The Daily Caller reported on Tuesday, Abedin and Mills appear to have given false statements about when they first learned of Clinton’s email use. But unlike Flynn, neither Clinton associate faced criminal charges.

Clinton herself was cleared of wrongdoing on July 5, 2016.

Johnson and Grassley, who chair the Senate Homeland Security and Senate Judiciary Committees, respectively, want to find out from Horowitz when the inspector general’s office first learned Strzok’s biased exchanges. They also want to know when Mueller’s office was informed.

In a statement issued over the weekend, a spokesman for Mueller said that Strzok was removed from the investigation “immediately upon learning of the allegations.”

Peter Carr, the spokesman, said that Page had completed a brief detail to Mueller’s team and had returned to the FBI “weeks before” the special counsel’s office was notified of the text messages.

Johnson and Grassley also asked Horowitz whether his office has received any other allegations about FBI and DOJ officials political bias.

Follow Chuck on Twitter