Paul Manafort, the former chairman of President Donald Trump's campaign, is seeking the dismissal of one of the two indictments against him, which focuses on financial crimes he allegedly committed while working for the Ukrainian government.

Manafort's lawyers say that because the charges do not relate to special counsel Robert Mueller's core mandate — investigating whether campaign officials colluded with Russia — they should be tossed out.

But Mueller's prosecutors indicated in court this week, and in previous filings, that the charges could be related to Russian collusion, after all.

US District Court Judge T.S. Ellis III pulled no punches while grilling prosecutors working for the special counsel Robert Mueller in federal court Friday.

But one of the prosecutors, Michael Dreeben, made a telling admission to Ellis near the end that offers an important clue about Mueller's case against Paul Manafort, the former chairman of President Donald Trump's campaign.

Friday's hearing was called to weigh Manafort's motion to dismiss one of the two indictments Mueller's office has brought against him. The indictment debated Friday was filed in Virginia and charges Manafort with tax fraud, bank fraud, and failure to report foreign bank accounts related to his lobbying work for the Ukrainian government and pro-Russian interests.

During Friday's hearing, Manafort's lawyers said that because the charges in the Virginia case do not relate directly to Mueller's core mandate — investigating Russian interference in the 2016 election and whether the Trump campaign colluded with Moscow — they should be tossed out.

When Dreeben countered and said the allegations in the Virginia case were within the scope of Mueller's mandate, Ellis replied, referencing one of the charges: "The scope covers bank fraud from 2005?"

"How does this have anything to do with the campaign?" Ellis asked, according to The Wall Street Journal. When Dreeben said Manafort had been in contact with individuals in Ukraine who are affiliated with Russia and pro-Russian interests, Ellis accused Dreeben of "running away from my question."

Dreeben said Mueller's office takes its "primary mission" of investigating Russia's election meddling and possible collusion with the Trump campaign "very seriously." He added that had prosecutors found evidence of criminal activity that was not necessarily related to Mueller's mandate, they would have referred it to another office.

According to The Journal, Ellis then pointed out that Mueller's office had made one such referral to the US attorney's office for the Southern District of New York regarding Michael Cohen, Trump's longtime personal lawyer.

Dreeben's statement to Ellis raises the possibility that the charges Mueller's office has leveled against Manafort that relate to his Ukraine lobbying work may also be connected to Russian collusion, legal experts said.

"Dreeben's statement that Manafort was in touch with Russians in Ukraine was responsive to the judge's question," said Jeffrey Cramer, a former federal prosecutor in Chicago who spent 12 years at the Justice Department.

"If prosecutors got information that Manafort was talking to relevant Russians — not just a Russian cabbie in Kiev — but relevant Russians, and then uncover evidence of criminal wrongdoing related to his Ukraine work as a result, there's your nexus," he added.

Manafort's spokesperson declined to comment.

His lawyers filed documents in court Monday which said the US government has not provided Manafort with evidence from wiretaps, statements, emails, or text messages to support its claim of a connection between Manafort and Russian individuals or Russian interests.

Prosecutors say they have long scrutinized Manafort's ties to Russian interests

Paul Manafort Chip Somodevilla/Getty Images

Friday was not the first time Mueller's team argued that the charges against Manafort are related to the special counsel's primary mandate of investigating whether the campaign colluded with Moscow.

In court documents filed last month, prosecutors told a federal judge in Washington, DC overseeing a separate criminal case against Manafort that they suspected Manafort of serving as a "back channel" between the campaign and Russia during the election.

When Manafort's lawyers argued at the time that Mueller's mandate does not cover Manafort's consulting work in Ukraine, Dreeben pushed back and said they were justified in probing Manafort because of his elevated role on the campaign and his "long-standing ties to Russia-backed politicians" in Ukraine.

"Did they provide back channels to Russia?" Dreeben said. "Investigators will naturally look at those things."

Mueller's office said in another April 2 court filing that any investigation into ties between the Trump campaign and Russia "would naturally cover ties that a former Trump campaign manager had to Russian-associated political operatives, Russian-backed politicians, and Russian oligarchs."

"It would also naturally look into any interactions they may have had before and during the campaign to plumb motives and opportunities to coordinate and to expose possible channels for surreptitious communications," it added. "And prosecutors would naturally follow the money trail from Manafort's Ukrainian consulting activities. Because investigation of those matters was authorized, so was prosecution."

Prosecutors point to a memo deputy attorney general Rod Rosenstein sent to Mueller in August 2017, three months after appointing him special counsel, as evidence that they were authorized to probe Manafort's work in Ukraine.

The memo outlines the full scope of Mueller's mandate, and though much of the document was redacted before being publicly released, it says Mueller is permitted to investigate at least two threads of criminal allegations related to Manafort:

Whether Manafort colluded with Russian government officials as Russia was trying to meddle in the 2016 US election.

Whether he committed any crimes "arising out of payments he received from the Ukrainian government before and during the tenure of President Viktor Yanukovych."

Cramer, the former federal prosecutor, said Saturday that as long as Mueller's team had a "reasonable purpose" for looking at Manafort's connections to Russians and Russia-backed individuals, it would be "perfectly legitimate" for them to follow any leads they came across as a result.

"If their initial look at Manafort and his Russia ties was within Mueller's scope — and it seems like that would be the case if the Russians were at all relevant — and they come across Manafort's allegedly illicit activities related to lobbying, not filing taxes, money laundering, and all that, any prosecutor would investigate that," Cramer said. "No question."

Ellis ordered Mueller's team on Friday to turn over an unredacted copy of Rosenstein's memo to the court under seal within two weeks.

Experts say it's unlikely Ellis will ultimately dismiss the charges against Manafort. But in the event that he does, the US attorney's office for the Eastern District of Virginia will likely pick up the thread.

"This is a scathing, detailed indictment against Manafort," Cramer said. "They've got him cold. The only question now is whether Mueller or the US attorney's office will charge him."