(CNN) Special counsel Robert Mueller's investigation into President Donald Trump and Russia was effectively on trial Monday at the first congressional hearing into the Justice Department watchdog report that faulted key decisions in the FBI's handling of the 2016 Hillary Clinton email investigation.

While the inspector general report last week did not probe the FBI's handling of the Russia investigation, Mueller's investigation loomed over the attacks that came from both Republicans and Democrats over the Clinton investigation.

Senate Judiciary Committee Chairman Chuck Grassley charged that the Justice Department has a "serious credibility problem," ticking off a number of purported differences between how the agency handled the Clinton and the Trump probes as a "double-standard."

"They see a story of kid glove treatment for one side and bare-knuckle tactics for the other," Grassley said.

But Sen. Dianne Feinstein, the top Democrat on the committee, countered that it was Clinton's campaign that was actually harmed by how the FBI handled the investigation when FBI Director James Comey announced publicly the FBI was re-opening the Clinton investigation after new emails were discovered on former Rep. Anthony Weiner's laptop.

"Both investigations were ongoing during the presidential election, but only the Clinton investigation was discussed publicly. This unquestionably harmed candidate Clinton and helped candidate Trump," Feinstein said.

Trump and his Republican allies have pointed to the anti-Trump text messages sent by FBI agent Peter Strzok and former FBI lawyer Lisa Page, who both were part of the Clinton and Trump-Russia investigations, to argue that the Mueller investigation is biased against Trump. Strzok was removed from Mueller's team last year after the texts were discovered.

Said Sen. Patrick Leahy, a Vermont Democrat: "Clearly some of Mr. Strzok's text messages were inappropriate, but if the FBI were trying to throw the election to Hillary Clinton, they could not have done a worse job."

Justice Department Inspector General Michael Horowitz and FBI Director Chris Wray, who testified at Monday's hearing, tried to avoid weighing in on the Trump investigation or any other issues that stemmed from it.

But the Mueller probe was an unavoidable topic for the senators in both parties, as the inspector general report has become the latest flashpoint in the battle between Trump and his allies and the special counsel probe.

Horowitz reiterated the findings in his report released last week, which faulted Comey for being "insubordinate" and deviating from the bureau's norms in key decisions about the Clinton investigation.

Horowitz says political messages sent by FBI employees like Strzok and Page "cast a cloud" over the Clinton investigation and "sewed doubt" about the credibility of the FBI's handling of it, but also said that the report found no evidence that the decision to close the investigation without charging Clinton was based on "political bias."

Grassley noted that three FBI employees who worked on the Clinton investigation who sent inappropriate political messages -- including Strzok and Page -- were later part of Mueller's team.

The third employee, an unnamed attorney whose instant messages were detailed in the report, "did not work for the Special Counsel's Office as a prosecutor or as an investigator," according to Joshua Stueve, a spokesman for the Special Counsel's office.

"FBI Attorney 2 left the Special Counsel's investigation and returned to the FBI in late February 2018, shortly after the OIG provided the Special Counsel with some of the instant messages discussed in this section," according to the IG report.

Monday's hearing is the first of several stemming from the inspector general report, and Horowitz will appear in the House on Tuesday. Strzok himself appears likely to testify soon , as his lawyer told the House Judiciary Committee that he would voluntarily testify to tell his side of the story.

Republicans seized on the messages to push Horowitz on whether bias actually did affect the Clinton email investigation.

"I'm glad you found what you found, Mr. Horowitz. I'm not buying that the Clinton email investigation was on the up and up," said Sen. Lindsey Graham, a South Carolina Republican. "The lead investigator clearly did not want to see Donald Trump elected President of the United States. ... If they found Hillary Clinton was criminally liable, that paves the way for Donald Trump."

Texas Sen. John Cornyn, the No. 2 Senate Republican, said Horowitz's findings "call into question the credibility of the whole Clinton email investigation and cast a cloud over the Russia investigation."

"I share the concerns and we wrote in fact here that it did cast a cloud over the entire Clinton email investigation," Horowitz said.

"And the Russia investigation?" Cornyn responded.

"Well, we haven't reached a conclusion on that," Horowitz said.

But Democrats used the same testimony to rebut the President's claims that the inspector general report exonerated him and proved there was no collusion, which Trump tweeted last week after the report was released.

"We did not look into collusion questions," Horowitz told Leahy.

And Leahy pressed Wray to rebut Trump's "witch hunt" charge , asking the FBI director whether he had "any reason to believe this investigation has been discredited?"

"Senator, as I said to you last month and as I said before, I do not believe special counsel Mueller is on a witch hunt," Wray responded.

Senators in both parties also pressed Horowitz to provide details about additional investigations into FBI and the Justice Department actions taken during the 2016 election and into the Trump administration.

Horowitz did confirm to Grassley there was an open investigation into how Comey handled the memos he wrote memorializing his conversations with Trump before he was fired last year.

Democrats were focused on another possible investigation: whether there was an ongoing probe into alleged leaks from the FBI to Rudy Giuliani. But both Horowitz and Wray declined to discuss it.

Reminded by Leahy that as FBI director, Comey testified publicly that the agency in fact was conducting an internal investigation into leaks to Giuliani, Wray responded: "There are a number of things that I probably would have done differently."

It wasn't the only time Wray tried to distance himself from Comey. Asked about whether he would take a role in a charging decision, Wray said: "What I can tell you is that I cannot imagine a situation in which I would unilaterally assume for myself as the FBI director a charging decision, and then announce it in a news conference."

This story has been updated with additional developments.