FBI Special Counsel Robert Mueller’s Russia investigation is under intense scrutiny after it was revealed that he dismissed investigator Peter Strzok for anti-Trump texts he sent to a fellow employee — a revelation that has fueled concerns of bias at the FBI.

Here’s what you need to know about Strzok:

Strzok Was Demoted Over the Texts

Strzok, who the New York Times reported is “considered one of the most experienced and trusted F.B.I. counterintelligence investigators” and was a top investigator in Mueller’s Russia probe, was demoted to a human resources position after the discovery of the texts — which were sent to an FBI lawyer with whom he was having an extramarital affair.

Strzok Was Involved in Softening Comey’s Language on Clinton’s Email Use

CNN reported that Strzok was involved in changing the language FBI Director James Comey used in his July 2016 statement to describe Hillary Clinton’s handling of classified information. The language was reportedly changed from “grossly negligent” to “extremely careless.”

The subsequent language is softer and therefore was less damaging to Clinton — who was at that time locked in a bruising presidential campaign with then-candidate Donald Trump. CNN noted that federal law carries penalties for gross negligence in the handling of classified material.

Strzok Interviewed Michael Flynn, Top Clinton Aides

Strzok was reportedly involved in the interviews of Michael Flynn in January for the probe into Russian interference, as well as Clinton aides Huma Abedin and Cheryl Mills concerning the Clinton email scandal.

Those interviews are now being scrutinized as Flynn was charged for lying during that interview about his contacts with Russian officials, but Abedin and Mills were not charged for saying they did not know about Clinton’s private email server until 2013 — when there were emails from earlier suggesting they do.

Strzok’s roles in both interviews have stoked concerns about bias and double standards in the FBI’s probes.

Strzok Is Not the Only One

Strzok is not the only agent accused of having a left-leaning bias. Documents obtained by Judicial Watch show that the prosecutor now serving as Mueller’s top deputy sent an email in January gushing over then-Acting Attorney General Sally Yates’ refusal to enforce President Trump’s travel ban.

“I am so proud. And in awe. Thank you so much. All my deepest respects,” said Andrew Weissman in the email, one of a number of supportive emails Yates received from DOJ colleagues.

Congress Wants Answers

Strzok’s potential bias has caught the attention of Republicans in Congress. Rep. Peter King (R-NY) told the New York Post Tuesday that he and fellow members of the House Intelligence Committee want to review Strzok’s “text messages, e-mails and contacts.”

They also want to know his impact on the findings of the investigation into Clinton’s email use and why he was questioning Michael Flynn. He also said the FBI and Justice Department have been “stonewalling” them.

“I think we should definitely question him. In fact, he’s one of the people we’ve been trying to get information on for the last several months,” King said.

Adam Shaw is a Breitbart News politics reporter based in New York. Follow Adam on Twitter: @AdamShawNY.