Senate Judiciary Committee Chairman Chuck Grassley (R-IA) increased the pressure on FBI Special Counsel Robert Mueller Thursday, calling on him to be more transparent amid questions over anti-Trump text messages from a top FBI investigator.

“I’ve got some advice for Mueller, but I’ve still got confidence in the work he’s doing, because I know him as an individual,” Grassley told reporters, according to Bloomberg News. “But he could help himself a lot by being more transparent.”

Grassley is the latest Republican to pile more pressure on Mueller amid a growing controversy over instances of bias in the FBI and the Department of Justice. That controversy has centered around text messages sent in 2016 between top investigator Peter Strzok and fellow official Lisa Page, with whom Strzok was having an affair.

“I cannot believe Donald Trump is likely to be an actual, serious candidate for president,” Page texted Strzok in March 2016. In another, she describes Trump as a “loathsome human being.”

In August, Strzok texted Page, “I want to believe the path you threw out for consideration in [Deputy FBI Director Andrew McCabe’s] office that there’s no way he gets elected — but I’m afraid we can’t take that risk. It’s like an insurance policy in the unlikely event you die before you’re 40.”

Strzok, who was also a key investigator on the Hillary Clinton email probe, was dismissed from Mueller’s Russia “dream team” after Mueller was informed about the messages. But the existence of the messages, particularly considering Strzok’s important role in both probes, has raised suspicions about impartiality.

“We are now beginning to better understand the magnitude of this insider bias on [FBI Special Counsel Robert] Mueller’s team,” Rep. Bob Goodlatte (R-VA) said at a House Judiciary Committee hearing Wednesday.

Strzok reportedly interviewed top Clinton aides Huma Abedin and Cheryl Mills and was behind the softening of the language used by then-FBI Director James Comey to describe Clinton’s handling of classified information — from “grossly negligent” to “extremely careless.”

On Thursday, Grassley noted that, while Mueller demoted Strzok months ago, “it’s just coming out now.”

“When you have conflict of interest, there’s going to be less embarrassment if you tell people right away, and there’s no reason for his investigation to be questioned except by the secrecy he maintains,” Grassley said, according to Bloomberg.

While he conceded that Mueller could not be too public about what he is investigating, he said that the lack of transparency over personnel has created problems “for a guy that has great prestige and integrity in this town.”

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