The US Democratic National Committee – which is suing Russia, the Donald Trump election campaign and WikiLeaks for interfering in the 2016 election – has suggested in court filings that the elusive Maltese professor at the centre of the campaign meddling investigation may actually be dead, according to the DNC’s lawyers.

The case, which is due to begin being heard on 13 September, is unrelated to Friday’s sentencing of Trump’s former foreign-policy adviser George Papadopoulos for lying to investigators about his contacts with Joseph Mifsud and the Russians.

The DNC said in a court filing on Friday that all the defendants in the case have been served with the complaint, “with the exception of Mifsud (who is missing and may be deceased).”

The filing did not offer any explanation as to why the Committee thinks the Russia-linked Maltese professor may not be alive, but noted that it “continues to monitor news sources for any signs of Mifsud’s whereabouts and will attempt service on Mifsud if and when he is found alive”.

The lawyers did not elaborate further in the court filing as to why they think Mifsud may be dead.

A hearing on the DNC lawsuit is scheduled for 13 September in the Manhattan federal court.

The suggestion that Mifsud may be dead was made on Friday, the very same day that Trump’s former foreign-policy adviser George Papadopoulos (above) was sentenced to two weeks in jail for lying to investigators about his contacts with Mifsud, who has been suspected of peddling dirt from Russian officials about Hillary Clinton.

In a sentencing memo filed on Friday by Special Counsel Robert Mueller, charged with investigating the Trump-Russia collusion accusations, explains how the defendant (Papadopoulos) had identified Professor Mifsud “only after being prompted by a series of specific questions about when the defendant first learned about Russia’s disclosure of information related to the campaign and whether the defendant had ever ‘received any information or anything like that from a Russian government official”.

Mueller wrote: “While denying he received any information from a Russian government official, the defendant identified the Professor by name – while also falsely claiming he interacted with the Professor ‘before I was with Trump though’.

Mueller goes on to describe how, “Over the next several minutes in the interview, the defendant repeatedly and falsely claimed that his interactions with the Professor occurred before he was working for the Trump campaign, and he did not mention his discussion with the Professor about the Russians possessing ‘dirt’ on Clinton. That fact only came up after additional specific questioning from the agents.

Mueller writes that Papadopoulos said in the interview: “I wasn’t working with Trump at the time. I was working in London . . . with that guy [the Professor].”

“Only then,” according to Mueller, “after acknowledging that the agents had ‘brought this up’ and lying about when he received the information, did the defendant admit that the Professor had told him ‘the Russians had emails of Clinton’.”

The defendant, Mueller says, “hoped to convince the agents that his conversation with the Professor was simply a ‘strange coincidence’ and had nothing to do with his position in that campaign.

Mueller also writes how, “The defendant also made false statements in an effort to minimize the extent and importance of his communications with the Professor, including by referring to the Professor as ‘a nothing’ who was ‘just a guy . . . talk[ing] up connections or something,’ even though the defendant understood the Professor to have substantial connections to high-level Russian government officials and that the Professor spoke with some of those officials in Moscow before telling the defendant about the ‘dirt’.

“The defendant’s lies were not only deliberate, but repeated. On at least a dozen occasions during the interview, the defendant falsely insisted that his interactions with the Professor took place before the defendant joined the Trump campaign. At various points during the interview, the defendant said the interactions took place ‘prior to even talking to anybody on Trump’.

Mueller quoted the defendant as saying: “This isn’t like [the Professor’s] messaging me while I’m with Trump or something.” The defendant also claimed repeatedly that his interactions with the Professor were a ‘very strange coincidence’.

Mueller adds: “And to further conceal his communications with the Professor, the defendant told yet another lie: as the agents asked the defendant additional questions about his communications with the Professor, the defendant assured them he was being truthful and transparent: ‘I’m just trying to, I’m giving you even the most minute transparent details’.”

But those lies, according to Mueller, “were material to the investigation”.

“The defendant’s lies to the FBI in January 2017 impeded the FBI’s investigation into Russian interference in the 2016 presidential election.

“Most immediately, those statements substantially hindered investigators’ ability to effectively question the Professor when the FBI located him in Washington, DC approximately two weeks after the defendant’s January 27, 2017 interview. The defendant’s lies undermined investigators’ ability to challenge the Professor or potentially detain or arrest him while he was still in the United States.

“The government understands that the Professor left the United States on February 11, 2017 and he has not returned to the United States since then. The defendant’s lies also hindered the government’s ability to discover who else may have known or been told about the Russians possessing ‘dirt’ on Clinton.

“Had the defendant told the FBI the truth when he was interviewed in January 2017, the FBI could have quickly taken numerous investigative steps to help determine, for example, how and where the Professor obtained the information, why the Professor provided the information to the defendant, and what the defendant did with the information after receiving it.”

Papadopoulos, the Trump campaign adviser who triggered the Russia investigation after conversations he had with Mifsud, was sentenced to 14 days in prison Friday after he told a judge he was "deeply embarrassed and ashamed" for lying to the FBI about his contacts with Russian intermediaries.

Papadopoulos, the first Trump campaign aide sentenced in special counsel Robert Mueller's ongoing investigation, acknowledged that his actions hindered an investigation of national importance, a move that the judge in his case said resulted in the 31-year-old putting his own self-interest above that of his country.

"I made a dreadful mistake, but I am a good man who is eager for redemption," Papadopoulos said.

The punishment was far less than the maximum six-month sentence sought by the government but more than the probation that Papadopoulos and his lawyers had asked for.

The sentence drew a quick response from Trump on Twitter on Friday, as he scoffed at the two weeks of prison time for Mifsud’s alleged accomplice by comparing it to an unverified cost figure for the Mueller probe: "14 days for $28 MILLION – $2 MILLION a day, No Collusion. A great day for America!"