Two top Republican chairmen stressed Friday that the final report from special counsel Robert Mueller "must be trusted by Americans."

In closing out their joint committee investigation into decisions made by the Justice Department, Oversight Chairman Trey Gowdy, R-S.C., and Judiciary Chairman Bob Goodlatte, R-Va., took a shot at their Democratic counterparts for playing up concerns that their probe was a campaign to undermine Mueller.

"Contrary to Democrat and media claims, there has been no effort to discredit the work of the Special Counsel," Gowdy and Goodlatte wrote in a letter to Senate Majority Leader Mitch McConnell, R-Ky., acting Attorney General Matthew Whitaker, and the DOJ inspector general. "Quite the opposite, whatever product is produced by the Special Counsel must be trusted by Americans and that requires asking tough but fair questions about investigative techniques both employed and not employed.”

The two retiring Republicans said their committees "have assiduously avoided interfering with any ongoing criminal, counter-intelligence, administrative, or judicial reviews of conduct engaged in 2016 or 2017."

The joint endeavor began in October 2017, looking into decisions made by the DOJ related to FBI's investigation into Hillary Clinton's email server, inquiry into Russian interference and possible collusion by the Trump campaign, and alleged surveillance abuse and bias by federal agents.

While President Trump has dubbed Mueller's investigation a "witch hunt" and called it illegitimate, Democrats had accused Gowdy and Goodlatte's effort of being a partisan effort to hurt the special counsel.

"House Republicans have failed to substantiate their allegations of 'FISA abuse' at every turn. In some cases, those allegations have been made in direct coordination with the White House," Rep. Jerrold Nadler, D-N.Y., said in a statement in March. "These are blatant attempts to distract from and undermine the credibility of Special Counsel Mueller. Chairman Goodlatte and Chairman Gowdy are simply off base — just as they were last year, when they called for a new special counsel to investigate a slew of Hillary Clinton conspiracy theories. Where there is no crime, there is no criminal investigation for a second special counsel to manage.”

Nadler is poised to take over the Judiciary Committee when Democrats take control of the lower chamber next week and has joined the chorus of incoming leaders talking up the investigations they plan to focus on Trump, which they say the GOP has neglected.

While Gowdy and Goodlatte defended Mueller, they did criticize the Office of the Special Counsel for denying or delaying Congress access to information citing an "ongoing investigation" in their letter and mentioned one instance in which it retrieved documents that were in congressional possession claiming they were improperly disseminated.

Gowdy and Goodlatte's investigation is not likely to survive into the next term with Democrats in charge; their best hope in the legislative branch is with Republicans in the Senate who still have the power to direct the investigative agenda.

The Democrat likely to take command of the House Intelligence Committee, Rep. Adam Schiff, chided the GOP duo's effort in a tweet Friday evening. "This is how the House Republican effort to undermine Mueller by 'investigating the investigators' ends. Not with a bang, but with a Friday, buried-in-the-holidays whimper, and one foot out the door," Schiff, D-Calif., wrote.

Upon the conclusion of interviewing 19 "key" witnesses and reviewing thousands of pages of documents, they renewed their call for a second special counsel — following the appointment of John Huber to investigate claims of FBI misconduct and an ongoing DOJ inspector general review.

Among their conclusions, Gowdy and Goodlatte concede they didn't obtain sufficient evidence to conclude whether a case could be brought against Clinton for her use of an unauthorized email server to conduct government business while serving as secretary of state. "To be clear, neither of us is in a position to know whether an investigation centered on the actual elements of the offense, addressing appropriate questions to witnesses with knowledge, or waiting until the end of the interview process — as opposed to May of 2016 — to draw conclusions would have resulted in a chargeable or prosecutable case," they wrote. "What we can say with confidence is the manner in which this investigation was conducted ensures we may never know the answers to those seminal questions."

Gowdy and Goodlatte also lamented the "institutional protectionism" at the DOJ and FBI, specifically regarding the issue of providing witnesses and documents in a timely manner.

"[C]onfidence in venerable institutions like the Department of Justice and the Federal Bureau of Investigation must be restored so the public can trust these institutions to make decisions solely on the facts and the law and totally devoid of political bias or consideration," they wrote.

In a separate letter to Whitaker and FBI Director Christopher Wray, Goodlatte urged that interview transcripts be released following a declassification process.