President Trump is taking up a common refrain of his allies who are critical of special counsel Robert Mueller's Russia-focused probe: Investigate the investigators.

Trump on Monday morning added extra emphasis to the refrain, tweeting about Mueller, 2016 Democratic rival Hillary Clinton and others, "Mueller, and the A.G. based on Mueller findings (and great intelligence), have already ruled No Collusion, No Obstruction. These were crimes committed by Crooked Hillary, the DNC, Dirty Cops and others! INVESTIGATE THE INVESTIGATORS!

The investigate-the-investigators mantra follows a series of official actions in that direction recently.

During testimony last week before a House committee, Attorney General William Barr said he is putting together a team to examine the Department of Justice's original probe in 2016 about contacts between the Trump campaign and Russia elements — including whether the DOJ, then under Obama administration control, improperly pried into Trump campaign matters.

"I want to make sure there was no unauthorized surveillance,” Barr told lawmakers. “The question is whether it was adequately predicated. And I'm not suggesting it was not adequately predicated. But I need to explore that,” Barr said.

Barr’s use of the term “spying” drew outrage from many Democrats.

Trump’s comments come amidst an independent investigation into possible FISA abuse and other Justice Department actions by DOJ Inspector General Michael Horowitz. When it was launched in March 2018, DOJ’s inspector general said it would “examine the Justice Department’s and the FBI’s compliance with legal requirements, and with applicable DOJ and FBI policies and procedures, in applications filed with the U.S. Foreign Intelligence Surveillance Court.” Barr said the investigation will conclude by May or June.

And U.S. attorney John Huber was also tasked by then-Attorney General Jeff Sessions in March 2018 with looking into the actions by the DOJ and FBI as well. The progress that Huber’s office has made, if any, is not known.

Meanwhile, Devin Nunes, R-Calif., the ranking member of the House Intelligence Committee, has also been putting together a series of referrals of Justice Department and FBI officials for months. Nunes is trying to meet privately with Barr about the referrals.

And in recent weeks, House Judiciary Committee ranking member Doug Collins, R-Ga., has released transcripts of the private interviews of former FBI agent Peter Strzok, former FBI lawyer Lisa Page, Justice Department official Bruce Ohr, his wife and former Fusion GPS contractor Nellie Ohr, former Trump campaign aide George Papadopoulos, former top FBI official Bill Priestap, and former FBI general counsel James Baker. Collins and fellow House Republicans suggest institutional plotting against the Trump campaign at the highest levels of federal law enforcement.

Republicans have criticized the way the DOJ and FBI conducted themselves during the Trump-Russia investigation, pointing to, among other the things, the use of the unverified so-called Trump Dossier by the FBI to obtain FISA warrants to spy on onetime Trump campaign official Carter Page. That dossier, compiled by British ex-spy Christopher Steele during his time working for Fusion GPS, was funded in part by Hillary Clinton's campaign and the Democratic National Committee through the Perkins Coie law firm. The dossier was given to more than a dozen members of the media and found its way into the DOJ and FBI through various individuals.

Democrats have countered that the FBI acted appropriately in obtaining the authority to surveil Trump campaign associates over concerns about Russian influence. In its rebuttal to the House Intelligence GOP memo, Democrats said the DOJ and FBI "met the rigor, transparency, and evidentiary basis needed to meet FISA's probable cause requirement."

Mueller’s report — with Barr’s redactions — is expected this week. Congressional Democrats want the entire unredacted, 400-page document. A court fight may be looming over access to all the materials.