WASHINGTON — As the Justice Department's internal watchdog prepares to release a long-awaited report examining the FBI's conduct in 2016 and 2017 in the Russia investigation, Democrats are expressing frustration over what they view as his failure to examine the conduct of Donald Trump's attorneys general over the past two years.

While inspectors general at other major cabinet agencies have conducted high profile investigations of Trump appointees, Democrats say the Justice Department's Michael Horowitz — appointed by President Obama in 2012 and confirmed by the Senate — has largely shied away. Trump's three attorney general appointees — Jeff Sessions, Matthew Whitaker and William Barr — have each escaped serious scrutiny from an inspector general who investigated Eric Holder, Obama's first attorney general, and many of his top deputies. There are two pending investigations that could involve decisions by high level Trump officials, but so far, it’s a record that puzzles his allies and infuriates critics.

"I don't have so much of a problem with Horowitz investigating some of the allegations surrounding the 2016 election, because that's his job," said Matthew Miller, a Democratic former DOJ spokesman and NBC News legal analyst. "But it is striking to me that with all of Barr's known misconduct, all of the instances of conversations between senior leadership and the White House, there doesn't seem to have been a single investigation into any that."

Barr disputes that he has engaged in misconduct. Congressional Democrats argue he has done the political bidding of the president and has improperly discussed sensitive cases with the White House, including the special counsel's Russia probe. They have also questioned the premise of the ongoing criminal investigation that Barr commissioned into its origins.

Democrats have asked the IG to examine Barr's handling of the Mueller report, including his decision to release a four-page letter that Democrats say mischaracterized the report's findings. But the inspector general said he lacked jurisdiction, because judgments by DOJ lawyers are a matter for the Office of Professional Responsibility, a lawyer discipline unit that reports to the attorney general.

"As far as we can tell, Horowitz has given a full pass to Trump's DOJ, the most politicized Department in modern times," said a senior Democratic Congressional aide who was not authorized to be quoted by name.

In a statement, Stephanie Logan, a spokeswoman for the Office of the Justice Department’s Inspector General, said the office "carefully considers every request for oversight made by any member of Congress."

"For example, Inspector General Horowitz has met directly with members of Congress and their staffs on multiple occasions to discuss their concerns about DOJ compliance with its policy on communications with the White House, and related matters. At each of these meetings, the inspector general has also flagged a limitation in the Office of the Inspector General’s jurisdiction, imposed by Congress, that restricts our ability to investigate allegations of professional misconduct by Department attorneys. We hope the Senate will consider the Inspector General Access Act, which the House passed on a bipartisan basis, and would resolve this jurisdictional limitation."

While the inspector general can investigate whether Justice Department rules and policies have been followed, a separate Justice Department entity — the Office of Professional Responsibility — is tasked with investigating issues of legal ethics, legal judgment and prosecutorial discretion. That office is not independent, and in fact reports to the attorney general.

Clinton, Comey and McCabe

Legal observers say Horowitz has a good reputation as a non-partisan investigator.

"Though I have disagreed with conclusions drawn by the inspector general and his office on various matters over the years, I have known Michael Horowitz for a long time and I like him, trust him, and respect him, and absolutely believe him to be a non-partisan professional," said Chuck Rosenberg, an NBC News analyst, former federal prosecutor and former head of the Drug Enforcement Administration.

Still, his record so far contrasts with that of other inspectors general in the Trump administration, some of whom have vigorously probed Trump appointees.

For example, the State Department's inspector general has investigated alleged bullying of career diplomats by Trump appointees. The Treasury Department's watchdog has examined whether IRS officials were pressured not to comply with a congressional request for Trump's tax returns. The Agriculture Department has probed allegations of tampering with scientific reports on climate change.

Horowitz, meanwhile, has issued major reports on the FBI's handling of the Hillary Clinton email investigation, on former FBI Director James Comey's dealings with the president, and on former deputy director Andrew McCabe's alleged lack of candor to investigators during the Obama administration. But he has issued nothing about Sessions, Barr or their top aides.

It's not for lack of requests. Democrats have asked the inspector general to look into whether Barr has been pressured by the president or others to open criminal investigations, and whether Sessions violated his recusal from the Mueller investigation.

There were also calls for him to examine possible political interference into DOJ antitrust decisions, including Trump's alleged opposition to the proposed merger of AT&T and Time Warner, which owns CNN.

Congressional Democrats asked him to investigate the appointment of Matthew Whitaker in the acting attorney general role without Senate confirmation and the Justice Department's role in the creation and implementation of the Trump administration's "zero-tolerance" family separation policy at the border. They have also called for, though not formally requested, a review of the FBI's investigation into complaints of sexual assault by Supreme Court Justice Brett Kavanaugh.

He is in fact examining zero tolerance, according to officials familiar with the matter, although the Department of Homeland Security's inspector general finished its first report on the same topic in September 2018. Horowitz is investigating at least three other issues at the request of Democratic House members and senators — the decision to abandon plans for a new FBI headquarters, allegations of systematic harassment of women working for the Bureau of Prisons, and the Drug Enforcement Administration's cooperation with local law enforcement.

But in response to most of those requests, the inspector general cited reasons he could not conduct them. In the case of the Sessions recusal, he argued it fell under the jurisdiction of Robert Mueller. In other cases he said he lacked jurisdiction, as when Sen. Sheldon Whitehouse of Rhode Island asked in February that Horowitz examine the Department's procedures for assessing conflicts of interest of senior political appointees.

"Horowitz has done a great job of navigating politically fraught and complicated issues. But if it’s true, that would be an interesting imbalance if the preponderance of their work is what the executive branch is asking them to look into rather than what Congress is looking into," said Danielle Brian, executive director of the Project on Government Oversight, a nonpartisan independent government watchdog. "I think it is as important for any IG to be tending to congressional requests. They're as important a constituent of his as the executive is."

Many of the IG's audits and investigations stem from internal priorities rather than Congressional requests. In October, Horowitz drew praise from Democrats when he co-authored a letter, signed by more than five dozen inspectors general, that sharply criticized the Justice Department's determination that the whistleblower complaint in the Ukraine scandal was not an "urgent concern" that needed to be forwarded to Congress.

But multiple Democratic Congressional aides told NBC News there was a widespread perception in their camp that Horowitz is not aggressively holding the Justice Department's political leaders accountable for their questionable actions.

Horowitz was appointed to the job by Obama in 2012. A New York native, he worked as a federal prosecutor in the Southern District of Manhattan in the 1990s, serving as Chief of the Public Corruption Unit from 1997 to 1999.

From 1999 to 2002, he worked at Justice Department headquarters in Washington, first as a deputy assistant attorney general, then as chief of staff for the criminal division.

In 2002, he became a partner at the firm of Cadwalader, Wickersham & Taft, where he focused on white collar defense, internal investigations, and regulatory compliance.

During that time he served as a commissioner on the U.S. Sentencing Commission — a position for which he was confirmed by the Senate in 2003.

In his years as inspector general, he developed a reputation as a bipartisan and fair investigator, if not a beloved one.

Barbara McQuade, an NBC News legal analyst and former U.S. Attorney under Obama, said, "From what I know he has a good reputation…Everyone who knew him always thought he was a good, solid prosecutor. I don't have any reason to believe he is politically motivated in any way."

Walter Schaub, former White House Office of Government Ethics director, said, "I've worked with Horowitz and have a great deal of respect for him. My experiences with him have all been very good. Hopefully he'll have an explanation for those statistics."

Among Horowitz's most high-profile investigations during the Obama administration was his review of what is shorthanded as the ATF gunwalking scandal — an operation by the Bureau of Alcohol, Tobacco and Firearms, code-named Fast and Furious, that went badly wrong when guns purchased in undercover operations were later used in crimes.

That case began amid demands from a Republican senator, Charles Grassley of Iowa—although Attorney General Eric Holder ultimately asked the IG to take a look. Grassley in 2018 joined with Republican Sen. Lindsey Graham of South Carolina to request investigation into national security surveillance of Trump campaign aide Carter Page. The IG agreed to investigate, and it morphed into the report he expects to release this month.

In 2013, Horowitz published the result of an investigation into the Voting Rights Section of Civil Rights Division, prompted by Republican complaints about the Obama Justice Department's handling of civil complaints involving alleged voter intimidation by members of the New Black Panther Party in Philadelphia.

It was just the kind of politically fraught investigation Democrats say Horowitz is now avoiding. In the end, he concluded that while "numerous witnesses told us that they believed that improper partisan or racial considerations have infected enforcement decisions in Voting Section cases at various times since 2001…our review generally did not substantiate the allegations we heard about partisan or racial motivations and did not support a conclusion that the Voting Section has improperly favored or disfavored any particular group of voters in the enforcement of the Voting Rights laws."

Now, Horowitz has told the Senate that Dec. 9 will be the release date for his long-awaited report on how the FBI used the Foreign Intelligence Surveillance Act and other investigative techniques during the 2016 presidential campaign. Horowitz is slated to testify before the Senate Judiciary Committee on Dec. 11.

Republicans have questioned the rationale for FISA surveillance of former Trump campaign aide Carter Page, and the genesis of the Russia probe itself. Amid reports that Horowitz determined that an FBI agent falsified records, some Democrats worry that any finding of misconduct by Horowitz, however tangential to the main issues in the Russia investigation, will allow Trump to express vindication.

But reports in the Washington Post and New York Times over the weekend suggest that in the big picture, Horowitz determined that political bias did not taint the Russia investigation, refuting longstanding allegations by President Trump and his allies.