Joseph Mifsud, the mysterious Maltese professor believed to have played a key role in igniting the Russia probe, vanished from public eye after his name began surfacing in news stories.

Now, lawyers for the Democratic National Committee say it is possible Mifsud is dead.

In a filing in the U.S. Southern District of New York, the DNC said Mifsud “is missing and may be deceased.” The lawyers said they will “monitor news sources” for any indication of Mifsud’s whereabouts and “will attempt service on Mifsud if and when he is found alive.”

The filing came as the DNC is suing Mifsud and others as part of its lawsuit accusing Trump officials of conspiring with Russia in the 2016 election.

The DNC’s lawyers did not elaborate in the filing on why they believe Mifsud may be dead. But a committee spokeswoman said lawyers haven’t been able to locate him, unlike other defendants in the lawsuit.

“The DNC's counsel has attempted to serve Mifsud for months and has been unable to locate or contact him,” DNC spokeswoman Adrienne Watson told Fox News. “In addition, public reports have said he has disappeared and hasn't been seen for months."

But there’s already been pushback to the DNC’s claim: The Daily Caller News Foundation interviewed Stephan Roh, a close friend of Mifsud, who said “really good sources” told him Mifsud is “alive, that he has another identity, and that he is staying somewhere, at a nice place.”

While Mifsud is far from a household name, investigators say he was the one who told then-Trump campaign aide George Papadopoulos in April 2016 that the Russians had “dirt” that could damage Hillary Clinton’s 2016 presidential campaign.

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Over drinks in London, Papadopoulos then told Australian diplomat Alexander Downer about his conversations with Mifsud. As the story goes, Downer informed U.S. officials, leading the FBI to open its Russia investigation during the 2016 election.

It has long been suggested -- in court documents filed by Special Counsel Robert Mueller’s team, by Democrats on Capitol Hill and in the media -- that Mifsud may have been connected to Russian intelligence, though some have noted his ties to Western institutions to challenge that narrative.

On Friday, Papadopoulos was sentenced to 14 days in prison after pleading guilty to lying to the FBI.

According to prosecutors, Papadopoulos' lying prevented the FBI from interviewing Mifsud.

Papadopoulos could not be reached for comment Monday. But in court Friday, he said, “My entire life has been turned upside down, I hope to have a second chance to redeem myself.”

SPECULATION RAGES OVER MIFSUD’S ROLE: WHO WAS MALTESE PROF WORKING FOR?

Court documents from the special counsel’s office have referred to Mifsud as “an overseas professor” with “substantial connections to Russian government officials.”

The Democrats on the House Intelligence Committee, in their report on Russia’s attempted interference in the election released in April, described Mifsud as “Kremlin-linked.”

In any case, Mifsud has become an important figure in Mueller’s probe.

A February memo released by the Republicans on the House Intelligence Committee said the intelligence passed on by the Australians about the diplomat’s meeting with Papadopoulos “triggered the opening of an FBI counterintelligence investigation in late July 2016 by FBI agent Pete Strzok.”

Fox News’ Jake Gibson and Adam Shaw contributed to this report.

EDITOR’S NOTE: A previous version of this report used an incorrect photo in depicting Professor Joseph Mifsud.