Former senior intelligence and FBI officials who are cooperating with the U.S. Justice Department's criminal probe into whether former CIA Director John Brennan lied to Congress were subpoenaed over the weekend to testify before a grand jury in Washington, D.C., multiple sources familiar with the matter told CBS News.

The subpoenas went out shortly after the Justice Department appointed conservative Trump ally Joe DiGenova to formally take over the criminal investigation into Brennan after the career prosecutor handling the case was removed late last week, CBS previously reported.

DiGenova is a staunch loyalist of President Trump who previously represented his campaign in its failed efforts to overturn the results of the 2020 presidential election. 

The cooperating witnesses are scheduled to appear before the grand jury as soon as this week, sources said. Many of them have been cooperating with the probe and were already scheduled to sit down for interviews with FBI agents and prosecutors before the new grand jury subpoenas came out over the weekend. 

Although the witnesses are agreeing to meet voluntarily with DOJ investigators, that does not necessarily mean their testimony will help advance the department's criminal case.

An attorney for Brennan denied any wrongdoing. 

The decision to abruptly reschedule the voluntary interviews from cooperators and ask for in-person grand jury testimony is unusual, legal experts told CBS. Customarily such interviews are conducted outside of the grand jury, and agents later provide details from them to grand jurors about what the witnesses told them under oath.

The move could be an effort by DiGenova to use every avenue possible, after grand jurors in Washington D.C. have increasingly declined to indict targets in cases that were widely perceived as politically motivated.

Until DiGenova came on board, the case was being overseen by Maria Medetis Long, a career prosecutor in Miami who oversees the office's national security matters.

Late last week, she informed lawyers for witnesses in the case that she was being removed, without any explanation. A source familiar with the matter told CBS that she was removed after she raised concerns about the strength of the evidence in the case.

A Justice Department spokesperson confirmed the personnel shift, calling it "healthy and normal," a characterization that former prosecutors say is false and contrary to standard operating procedures.

A similar type of concern came about last year, when Mr. Trump ousted the former top federal prosecutor in Virginia's eastern district and replaced him with loyalist Lindsey Halligan after he raised concerns about the strength of the evidence against former FBI Director James Comey and New York Attorney General Letitia James.

Halligan secured indictments against both Comey and James, but they were quickly dismissed after a federal judge ruled that Halligan was unlawfully appointed.

The probe into Brennan was sparked by a referral from the Republican-led House Judiciary Committee last October over allegations that Brennan lied to Congress about the CIA's role in crafting the intelligence assessment into Russia's efforts to meddle in the 2016 presidential election.

In the referral, Rep. Jim Jordan, the committee's chairman, claimed that in 2023 testimony, Brennan "falsely" denied the CIA relied on a dossier prepared by former British intelligence officer Christopher Steele during the drafting of the intelligence assessment and falsely told the committee the CIA had opposed including the Steele dossier in the assessment.

The so-called Steele dossier contained salacious allegations against Mr. Trump, who was then still a candidate, that have not been verified.

One witness in the case, a former CIA official, was supposed to be interviewed for the third time in early May, before the new grand jury subpoenas were issued.

The witness previously answered questions concerning Brennan's 2023 congressional testimony, and offered no information that contradicted Brennan's account, one of the sources told CBS. 

The official has also been questioned about a disagreement he had with Brennan over whether there was enough intelligence to assess that Russia was trying to help Mr. Trump and harm his rival, Democratic Secretary of State Hillary Clinton. 

It is unclear whether that dispute will factor into the false statements probe.

The U.S. attorney's office in Miami is also investigating a separate referral from Director of National Intelligence Tulsi Gabbard in which she claimed, without evidence, that Brennan and other Obama-era officials "manufactured" the 2017 assessment.

The status of that probe, which defense lawyers refer to as the "grand conspiracy" case, remains unclear, though DiGenova is expected to be involved in that one as well.