Special counsel Robert Mueller's glowing performance review of star witness and former national security advisor Michael Flynn suggests there are more shoes to drop in the ongoing probe of Russian election interference.

In a heavily redacted court filing ahead of Michael Flynn's Dec. 18 sentencing, Mueller on Tuesday recommended a light sentence — possibly no incarceration — for President Donald Trump's former aide.

Mueller said in the sentencing memo that Flynn, 60, had provided "firsthand insight" over the course of 19 interviews with special counsel investigators and Justice Department attorneys, and that his benefits to their work "may not be fully realized" until the investigations are finished.

The filing reveals that Flynn's cooperation extended beyond the probe of Russian meddling in the 2016 U.S. presidential election and possible coordination between Trump campaign-related figures and the Kremlin.

One lengthy section, almost entirely blacked out, points to a separate and unspecified "criminal investigation" in which Flynn had "provided substantial assistance" to investigators.

"Mueller still has many more revelations to drop, and they're going to land hard," former federal prosecutor Elie Honig wrote in a CNN op-ed about the memo.

White-collar attorney Sol Wisenberg, who was deputy independent counsel in the Whitewater investigation into then-President and first lady Bill and Hillary Clinton, said on Fox News that "the memo itself is kind of a dud," but only because "all the good stuff is redacted."

Flynn pleaded guilty in December 2017 to a single count of lying to the FBI about his contacts with Sergey Kislyak, Russia's ambassador to the U.S. Flynn spoke to Kislyak during the presidential transition period about sanctions slapped on Russia by President Barack Obama in December 2016, in retaliation for that country's attempts to interfere in the presidential election.

Some legal minds at the time considered the one-count plea agreement a "sweetheart deal" in light of previously revealed potential offenses Flynn was alleged to have committed, such as making false statements in documents he retroactively filed when registering as a foreign agent for Turkey. Others speculated more charges could have been unveiled against Flynn in Tuesday's sentencing memo.

"Flynn could have been subjected to additional charges that would have considerably raised that calculus," former U.S. Attorney Joyce Vance said Wednesday on MSNBC. But the fact that his plea continued to include just one charge of lying to the government "tells me that he really brought something valuable to the table," Vance added.

Lying to the FBI carries a possible maximum sentence of five years in prison. But Flynn's plea agreement with Mueller says he is eligible for a sentence ranging from zero to six months behind bars.

Before the memo was made public, professor David Shapiro of John Jay College of Criminal Justice said he would be looking to see why Flynn's plea deal gave that lenient sentencing range. "I would be seeking reasons why this case was both given intense scrutiny and pleaded downward," Shapiro said in an email. "'Where's the beef?'"

After reading the filing, Shapiro said, "It overwhelmingly compels the conclusion that more is to come" from Mueller.

Not everyone viewed the sentencing memo as an hors d'oeuvre on Mueller's full menu of criminal discoveries.

Rudy Giuliani, the former New York mayor who is representing Trump in the Mueller probe, reportedly sent a text to a Politico reporter appearing to derisively suggest that the filing produced nothing of value.

Rudy Giuliani via text to ⁦politico⁩ on tonight's Flynn filing: "Wow big crime for a SPECIAL WHATEVER maybe a group of Angry Bitter Hillary Supporters who are justifying themselves by the goal justifies the means....Over the top In ethical behavior."

Flynn's lawyers will file their own version of the sentencing memo by Dec. 11, followed by a reply from the government three days later.

The favorable treatment Flynn received from Mueller in the memo contrasts sharply with the special counsel's recent revelation that Paul Manafort, Trump's former campaign chairman, had lied to investigators after striking his own plea deal. Manafort was convicted in August on eight criminal counts related to work he did for a pro-Russia political party in Ukraine years before joining Trump's campaign. He pleaded guilty to other charges lodged by Mueller in September, just before his second trial began.

Mueller is expected to file another court document later this week in connection with Manafort.

Another former Trump associate, Michael Cohen, admitted last week that he lied to Congress about an aborted plan to build a Trump Tower in Moscow. Cohen, Trump's former personal attorney, has cooperated with Mueller's team in seven interviews reportedly totaling 70 hours of participation. He is expected to be sentenced on Dec. 12.

But while those matters are among the recent developments to be made public in the Mueller probe, it's not likely that Flynn had dished to the special counsel about them in particular, according to David Weinstein, a defense attorney and former federal prosecutor.

"The cooperation does not appear to be against Manafort or Cohen or it wouldn't have been blacked out," Weinstein said of the sentencing memo.

"There is more to come."