Special Counsel Robert Mueller is expected to file a key memo on Tuesday that could shed light on the extent of former National Security Adviser Michael Flynn’s cooperation with federal prosecutors throughout the Russia investigation.

The deadline for Mueller’s office to file a memo recommending sentencing for Flynn is Tuesday. The document is likely to provide a federal judge with details of how valuable the retired U.S. Army lieutenant has been to the special counsel, at a time when the investigation reportedly is tying up "loose ends."

Flynn pleaded guilty in December 2017 to making false statements to the FBI with regard to his communications with Russian Ambassador Sergey Kislyak. In late 2016, the two allegedly spoke about the U.S. sanctions imposed on Russia. Flynn was fired from the White House on Feb. 13, 2017, after he misled Trump administration officials, including Vice President Pence, on his communications.

Flynn also admitted to making false statements about his work as an unregistered foreign agent to benefit the Turkish government. Flynn was under investigation by the Justice Department for that work when he became national security adviser.

But the special counsel’s team has delayed Flynn’s sentencing date for over a year, repeatedly saying that “due to the status” of the investigation, “the parties do not believe that this matter is ready to be scheduled for a sentencing hearing at this time.”

Questions also have been raised about the Flynn plea itself, considering a congressional Republican report earlier this year claimed fired FBI boss James Comey once testified that FBI agents did not think Flynn intentionally lied.

In September, the special counsel’s office officially scheduled Dec. 18 for Flynn’s sentencing.

An attorney for Flynn and a spokesperson for the special counsel declined to comment.

Mueller’s Tuesday filing is expected to shed light on the level of Flynn’s cooperation with federal prosecutors as they probe whether Trump campaign associates colluded with Russia to influence the 2016 presidential election.

As part of Flynn’s plea deal, he agreed to cooperate with Mueller’s office on “any and all matters” deemed relevant. The plea deal said Flynn’s cooperation may include answering questions, taking government-administered polygraph examinations, providing sworn statements and participating in “covert law enforcement activities.”

Mueller’s team has entered plea agreements with several Trump associates, including former Trump campaign chairman Paul Manafort, who the special counsel now alleges breached their plea deal by lying to investigators. Manafort was convicted on multiple counts of financial fraud over the summer in connection with work he completed in Ukraine as a political consultant.

Mueller’s team also entered a deal last week with former longtime Trump personal attorney Michael Cohen, after he pleaded guilty to making false statements to Congress regarding his description of the Trump Organization’s plans to build a Trump Tower in Moscow. Cohen told the Senate Intelligence Committee that discussions about the project with Russia ended in January 2016, prior to the Iowa caucuses, the first contest in the presidential primaries.

Cohen admitted in court last week that discussions about the Moscow Project did not end until June 2016, well after the presidential race got underway.

The Mueller-Cohen deal also involves cooperation with federal prosecutors.

Other than Flynn and Cohen, Mueller’s team charged former Trump campaign foreign policy adviser George Papadopoulos with making false statements to the FBI. He is currently serving a 14-day prison sentence.

Former senior Justice Department official James Trusty told Fox News that the guilty pleas related to making false statements “does not suggest a particularly successful investigation.”

“It’s heavy on false statements charges, and that doesn’t suggest that it’s moving in the right direction for the prosecution,” Trusty said. “It doesn’t suggest to me that they’re making a lot of headway on the substance of their investigation which is Russian collusion.”

He added: “As an ex-prosecutor, false statements are a very distant second place.”

But according to memos penned by Comey, President Trump tried to protect Flynn by asking Comey to let the investigation into his false statements go. Trump has denied asking Comey to drop the investigation, but that conversation between the fired FBI director and the president is reportedly one under scrutiny by the special counsel’s office as he investigates whether Trump attempted to obstruct the Russia investigation.

But Trusty said that during the FBI’s investigation of Hillary Clinton’s private email server, investigators and Comey “swore off even pursuing false statements charges,” quoting a recent Justice Department inspector general report.

“Are there two sets of rules?” Trusty said. “It’s a fair question that someone in the FBI or Justice Department ought to answer at some point.”

The Associated Press contributed to this report.