WASHINGTON (Reuters) – The special counsel examining alleged Russian interference in the 2016 U.S. presidential election removed a top FBI investigator from his team for exchanging text messages with a colleague that expressed anti-Trump views, two U.S. newspapers reported on Saturday.

The New York Times and the Washington Post identified the investigator as FBI agent Peter Strzok, the deputy head of FBI counter-intelligence. He was reassigned last summer to the FBI’s human resources department after the Justice Department’s inspector general began looking into the text messages, the papers said, quoting several unidentified people familiar with the matter.

A source familiar with the matter confirmed the reports Strzok was transferred to the human resources department over the politically charged text messages.

Strzok played a key role in the FBI investigation into former Secretary of State Hillary Clinton’s use of a private email server, the papers said.

During that probe and the 2016 presidential election, Strzok and an FBI colleague exchanged texts that disparaged then-Republican candidate Donald Trump and favored Clinton, his Democratic rival, the Washington Post said. The newspapers did not disclose details of the text messages.

Reuters was unable to reach Strzok for comment. The New York Times said that a lawyer for Strzok declined to comment, while the Washington Post said it repeatedly sought comment from Strzok, but received no response.

Mueller’s office confirmed Strzok’s removal, but did not elaborate on the cause.

“Immediately upon learning of the allegations, the Special Counsel’s Office removed Peter Strzok from the investigation,” spokesman Peter Carr said.

In apparent reference to the case, the Justice Department inspector general’s office said in a statement on Saturday that it was “reviewing allegations involving communications between certain individuals.”

The matter came up during a review that the Justice Department launched into the FBI’s decision to announce an inquiry into Clinton’s emails shortly before the November presidential election.

The statement provided no further details and it did not mention any individuals by name. The Department of Justice did not respond to a request for further comment.

The FBI was not immediately available for comment.

According to the newspapers, federal law enforcement officials became concerned that Trump and his supporters could use the exposure of the text messages to attack the credibility of Mueller’s investigation.

Mueller, a former FBI director, is looking into possible collusion between Trump’s campaign and Russia, which ran an influence operation aimed at swinging the vote to Trump over Clinton, according to three U.S. intelligence agencies.

Trump has criticized the FBI’s handling of the Clinton email investigation, initially citing it as his reason for firing former FBI director James Comey on May 9. Some lawmakers have called for Mueller to resign.