Amid the media focus on allegations of Trump campaign collusion with Russia and Robert Mueller's special counsel probe, the investigation by the inspector general of the Justice Department into the probe of Hillary Clinton's mishandling of classified information had been going on quietly for nearly a year until this month.

The investigation by Inspector General Michael Horowitz drew attention Dec. 2 when Horowitz acknowledged that Mueller was alerted to evidence of bias against Trump in text messages by one of his lead investigators, FBI agent Peter Strzok, who also played a key role in the Clinton investigation.

Now, with interviews with former Attorney General Loretta Lynch, former FBI Director James Comey and other key officials under his belt, Horowitz's review looms as a "potential landmine" for Mueller, writes Kevin Johnson in an analysis for USA Today.

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The significance of the internal probe was underscored in congressional hearings this month when FBI Director Christopher Wray and Deputy Attorney General Rod Rosenstein and others asked Republican Congress members to withhold judgment about allegations of bias within the FBI until the Horowitz review is completed.

House Judiciary Committee Chairman Bob Goodlatte, R-Va., told Rosenstein at a Dec. 13 hearing that Horowitz's investigation is "very important."

"It is very encouraging to us that (Horowitz) is doing what I think is good, unbiased work," Goodlatte said.

In Horowitz's review of the Clinton email investigation, authorities are examining whether the Justice Department and FBI followed established "policies and procedures'' when Comey publicly announced the bureau would not refer criminal charges in the case, despite determining Clinton and her aides had been "extremely careless," tranferring classified information through the secretary of state's private, unsecured email system.

In 2012, Horowitz recommended 14 federal law enforcement officials for discipline after reviewing President Obama's botched "Fast and Furious" operation, an anti-gun-trafficking scheme that allowed an estimated 2,000 firearms to fall into the hands of Mexican drug cartel enforcers.

He also led the 2015 review that found U.S. Drug Enforcement Agents posted in Colombia had engaged in sex parties involving prostitutes who were supplied by local drug cartels.

DOJ 'predisposed' to believe anti-Trump dossier

Pointing to the anti-Trump text message sent by Strzok to his paramour, FBI lawyer Lisa Page, Andrew McCarthy, a former assistant U.S. attorney, wrote in a National Review column he believes the infamous anti-Trump "dossier" funded by a lawyer representing the Hillary Clinton campaign and the Democratic National Committee was used by the Obama administration to obtain FISA surveillance authority to investigate Trump on allegations of "Russia collusion."

McCarthy argued the Obama-era FBI and Justice Department had great faith in the author of the dossier, former British spy Christopher Steele, because Steele had previously collaborated with the bureau on a major case.

In addition, McCarthy said, Steele was working on the Trump-Russia project with the wife of a top Obama Justice Department official, who was personally briefed by Steele.

"The upper ranks of the FBI and DOJ strongly preferred Trump’s opponent, Hillary Clinton, to the point of overlooking significant evidence of her felony misconduct, even as they turned up the heat on Trump," McCarthy argued. "In sum, the FBI and DOJ were predisposed to believe the allegations in Steele's dossier."

McCarthy said that "because of their confidence in Steele, because they were predisposed to believe his scandalous claims about Donald Trump, they made grossly inadequate efforts to verify his claims."

On Tuesday, Trump backed up former Republican congressman Jason Chaffetz' claim in an interview on the Fox News Channel's "Fox & Friends" that the dossier is "bogus."

Trump wrote: "WOW, @foxandfrlends 'Dossier is bogus. Clinton Campaign, DNC funded Dossier. FBI CANNOT (after all of this time) VERIFY CLAIMS IN DOSSIER OF RUSSIA/TRUMP COLLUSION. FBI TAINTED.' And they used this Crooked Hillary pile of garbage as the basis for going after the Trump Campaign!"

Chaffetz cited the Washington Examiner's report that that the FBI refused to reveal who funded the dossier.

While several Republican leaders are calling on Trump to fire Mueller, the president told a reporters posing the question Sunday, "No, I'm not."

In an interview with ABC's "This Week" Sunday, Sen. John Cornyn of Texas, the Senate's No. 2 Republican, said it "would be a mistake" for Trump to fire the special counsel.

Mueller probe 'off the rails'

Rep. Francis Rooney, R-Fla., in an interview Tuesday with MSNBC, said the Mueller investigation is "totally off the rails," citing the revelation of anti-Trump texts by Strzok, the connection of senior Justice Department official Bruch Ohr to the company that produced the dossier and the $600,000 donation from Clinton-related sources to the state-Senate campaign of the wife of assistant FBI Director Andrew McCabe.

Challenged by MSNBC host Hallie Jackson for calling the FBI and DOJ "off the rails," Rooney replied: "I think the American people have very high standards for our government agencies and to see people like these—

Jackson interjected: "Are the agencies not living up to the standards?"

Rooney answered: "Well, those aforementioned examples are really nerve-racking to me and undermine my confidence that the agencies don’t respect the Constitution and will not put the ends before the means."

Jackson commented: "That’s a pretty broad brush you're painting with."

Rooney said: "Yeah, but we have seen a lot of ends-before-the-means culture, both out of the Obama administration and out of Hillary Clinton, with her $84 million dollars of potentially illegal campaign contributions or the Clinton Foundation, Uranium One. People need a good, clean government."

Horowitz, in a recent letter sent to Sens. Chuck Grassley R-Iowa, and Ron Johnson, R-Wis., said his office obtained the Strzok texts after asking the FBI to produce communications from employees who worked on the Clinton investigation.

Sen. Johnson, in an interview last Thursday with the Fox News Channel's Tucker Carlson, charged the FBI is stonewalling his efforts to get to the bottom of allegations that political bias influenced the bureau's decision to exonerate Clinton.

Johnson is demanding that the FBI reveal who was behind the watering down of then-FBI Director James Comey's draft statement on the Clinton email probe.

But he's not getting any answers from the bureau.

"I have no idea why the FBI doesn't just come clean and provide this information to Congress. What are they trying to hide? What are they trying to cover up?" Johnson asked.

Johnson has given the FBI until Dec. 27 to provide him with answers.

He told Carlson the bureau is being used as a "political weapon."

"They don't believe they need to or they have to" turn over the information, he said. "They're not going to be compelled to, apparently."

Johnson, recalling the infamous IRS attack on conservative non-profits, said the American people are losing confidence in governmental institutions as they become more and more politicized.

"We have seen now with this Lois Lerner in the IRS politicizing the more fearsome government agency from the standpoint from most Americans, used it as a political weapon," Johnson said.

"We've seen the politicization of the FBI. So we are losing confidence in these institutions that should be bedrocks of our democracy."

In his correspondence with Horowitz, Johnson raised the question of when the bureau first learned of Strzok's anti-Trump text messages, noted the Wall Street Journal's Kim Strassel.

Strassel wrote: "Was it before Mr. Mueller was even appointed? Did FBI leaders sit by as the special counsel tapped Mr. Strzok? In any case, we know from the letter that the inspector general informed both Messrs. Rosenstein and Mueller of the texts on July 27, and that both men hid that explosive information from Congress for four months."

Strassel also reported that two confidantes of former FBI director James Comey who were interviewed in an investigation of Comey in 2016 regarding politicization of the Clinton email probe refused to be interviewed without signing unprecedented nondisclosure agreements that gave the FBI full authority to withhold the information from Congress.

The investigation by the Office of Special Counsel centered on whether or not Comey violated the Hatch Act, which restricts political activity by executive-branch officials.

The Office of Special Council is a federal agency, distinct from Robert Mueller's probe, that polices personnel practices.

In a letter last week to FBI Director Christopher Wray, Johnson demanded that Wray authorize the release of the full transcripts and other documents.