WASHINGTON — A memo filed in federal court for the District of Columbia on Tuesday confirmed that President Trump’s former top national security adviser, Michael Flynn, has been cooperating extensively with special counsel Robert Mueller’s investigation into Russian interference in the 2016 election. The document also revealed that Mueller is working on a separate criminal investigation, but it did not divulge any details about the focus of that probe.

The memo was a sentencing recommendation for Flynn that summarized his misconduct and subsequent work with prosecutors. It said Flynn had provided “substantial assistance” to Mueller’s team since pleading guilty to one count of making false statements to the FBI in November 2017. In the document, Mueller recommended that Flynn — due to his cooperation — receive the “low end” of the zero- to six-month sentence that could have come with his crime.

“Given the defendant’s substantial assistance and other considerations set forth below, a sentence at the low end of the guideline range — including a sentence that does not impose a term of incarceration — is appropriate and warranted,” Mueller wrote.

The seven-page sentencing memo was accompanied by a heavily redacted six-page addendum detailing the “significance and usefulness” of Flynn’s assistance. Mueller had requested that the addendum to be partially sealed because it contained what he described as “sensitive information about ongoing investigations.”

Robert Mueller and Michael Flynn (Photo illustration: Yahoo News; photos: AP)

Most of the blacked out portions of the addendum dealt with a criminal investigation that the document distinguished from the special counsel’s principal probe into coordination between Russia and the Trump campaign. While Mueller’s documents provided almost no detail about this mysterious investigation, they confirmed it was separate from the Russia probe and that it is continuing.

It is not clear from the documents whether the second investigation involves Trump. The addendum also included indications the other probe might involve prosecutors outside of Mueller’s office. Mueller wrote that, as part of his cooperation, Flynn “participated in 19 interviews” with the special counsel or “attorneys from other Department of Justice offices.”

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In addition to confirming the separate criminal investigation, one section of the addendum that noted Flynn has “assisted with several ongoing investigations” tantalizingly indicated there may be a third probe. The special counsel’s office did not immediately respond to a request for comment about how many investigations Flynn has assisted with.

Michael Flynn after his plea hearing on Dec. 1, 2017, in Washington. (Photo: Chip Somodevilla/Getty Images)

Details of Flynn’s cooperation had been hotly anticipated by close observers of the Mueller investigation. Since Flynn’s guilty plea one year ago, he has not appeared as a witness in any public proceeding related to the special counsel’s office and no information clearly derived from his cooperation surfaced in any court documents filed by the special counsel apart from status updates postponing his sentencing. Consequently, Tuesday night’s filings represented the first substantial update to Flynn’s part of the investigation in a full year.

In addition, the amount of disclosure provided by Flynn’s sentencing memo was a key signal of the status of Mueller’s investigation overall. A highly detailed picture of Flynn’s cooperation could have indicated that Mueller had few matters left that he wished to keep secret and the investigation was winding down, as many expected.

However, the heavy redactions in the documents appear to indicate Mueller is continuing to work on significant matters not currently known to the public.

The documents filed on Tuesday also provide some hints of where Mueller’s Russia probe is focusing. It said Flynn had “provided firsthand information about the content and context of interactions” between Trump’s presidential transition team “and Russian government officials.”

As an example, Mueller cited information Flynn provided prosecutors about his own contacts with Russia. Much of this portion of the document was heavily redacted, however.

Michael Flynn and Russian President Vladimir Putin, at center on back side of table, attend a 2015 event marking the 10th anniversary of RT, a Russian TV news channel, in Moscow. (Mikhail Klimentyev/Sputnik, Kremlin Pool/AP) )

A U.S. intelligence community assessment concluded Russia intervened in the 2016 election to help Trump. However, Trump has repeatedly dismissed Mueller’s probe as a partisan “witch hunt” and insisted there was “no collusion” between his campaign and the Kremlin. The president has also attacked other former allies who have cooperated with Mueller and accused them of trying to avoid punishment for their own independent misdeeds.

The White House and Trump’s legal team did not immediately respond to requests for comment about the Flynn memo.

The false statements Flynn pleaded guilty to making to the FBI occurred in an interview he had with agents in January 2017, the month Trump took office. At the time, Flynn was serving as the president’s national security adviser. In that interview, Flynn falsely denied asking Russian Ambassador Sergey Kislyak not to respond to President Barack Obama’s sanctions against the Kremlin for the election interference. Flynn was fired from the Trump administration in February 2017 after it was discovered he also lied about his contacts with Kislyak to Vice President Mike Pence.

Mueller’s memo also revealed that Flynn admitted to misrepresenting his lobbying work on behalf of the Turkish government. He was not charged for that crime.

Prior to his short stint in the White House, Flynn was a particularly high profile — and high volume — supporter of Trump. At the Republican National Convention in 2016, Flynn gave a speech where he led a memorable “lock her up” chant calling for Democratic candidate Hillary Clinton to be imprisoned for using a private email server to conduct official business during her time as secretary of state under Obama.

Flynn had a decorated military career for more than three decades that culminated in reaching the rank of lieutenant general and serving as the head of the Defense Intelligence Agency under Obama. He was forced out in 2014, and reports at the time reference critics citing both a “chaotic” management style and policy plans for the agency that caused friction with both his supervisors and his subordinates.

Flynn’s attorney, Robert Kelner, did not immediately respond to a request for comment about the sentencing memo. Flynn is due to be sentenced on Dec. 18.

Additional reporting contributed by Michael Isikoff

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