The federal judge overseeing the criminal case in Virginia against President Donald Trump's former campaign chairman Paul Manafort questioned Friday why special counsel Robert Mueller brought the charges against Manafort.

US District Court Judge T.S. Ellis III speculated that prosecutors charged Manafort to get him to flip against Trump.

Former Justice Department veterans were confounded by the comment and said it has no bearing on the validity of the case against Manafort.

US District Court Judge T.S. Ellis III, who is overseeing one of the federal criminal cases against President Donald Trump's former campaign chairman Paul Manafort, made an unusual comment Friday that confounded legal experts.

During a hearing in which Manafort was seeking the dismissal of one of the two indictments against him in special counsel Robert Mueller's investigation into Russian election meddling, Ellis speculated that prosecutors had charged him to get him to flip against Trump.

Manafort is the subject of two indictments from Mueller.

The first, brought in Washington, DC, accuses Manafort of money laundering, failure to register as a foreign agent, and making false statements to investigators. The other, brought in Virginia, charges him with tax fraud, bank fraud, and failing to report foreign bank accounts. Both indictments center largely around Manafort's lobbying work for the Ukrainian government and pro-Russian interests.

Manafort's lawyers argued Friday for the dismissal of the Virginia indictment, saying that the charges should be tossed out because they do not relate specifically to the Russia investigation and the question of whether the Trump campaign colluded with Moscow to tilt the 2016 presidential race in Trump's favor.

Ellis appeared receptive, at least in part, to the argument.

"I don't see what relation this indictment has with what the special counsel is authorized to investigate," Ellis told prosecutors working for Mueller. "You don't really care about Mr. Manafort's bank fraud. ... What you really care about is what information Mr. Manafort could give you that would reflect on Mr. Trump or lead to his prosecution or impeachment."

Ellis later quipped: "The vernacular is 'to sing.'"

'Whether prosecutors brought the case to flip Manafort is entirely irrelevant'

Legal experts and former Justice Department prosecutors immediately raised questions about Ellis's comments.

"The operative question here is whether Manafort committed the crimes he's accused of and whether Mueller had the authority to charge them," said Jeffrey Cramer, a longtime former federal prosecutor who spent 12 years at the Justice Department. "Whether prosecutors brought the case to flip Manafort is entirely irrelevant."

Cramer added, moreover, that it's "hardly shocking" when prosecutors charge defendants to gain their cooperation against bigger fish.

"This is happening in every court room across the country right now, in cases ranging from drug conspiracies to fraud to organized crime," he said. "And appropriately so, otherwise those who make the big decisions get away with it, while lower-level people serve time in jail."

Alex Whiting, a former federal prosecutor from Boston and Washington, DC, said as much.

"It is well-established that it is beyond a judge's authority to inquire into, or second-guess, a prosecutor's exercise of charging discretion unless there is some evidence that the prosecutor based the decision on improper factors," Whiting said.

Echoing Cramer's view that prosecutors build cases all the time against powerful figures by prosecuting lower-level actors who then cooperate, Whiting said "most people would agree that's what we want prosecutors to do: focus their work on higher-level criminal actors."

He added that he does not expect Ellis to use Mueller's motivation in bringing charges as the basis of any decision he makes, but that if he does, "he will be reversed."

Experts also pointed out that Mueller did what any other prosecutor would have done during the course of an investigation.

Mueller was appointed by deputy attorney general Rod Rosenstein. He is tasked with investigating whether the Trump campaign colluded with Moscow to tilt the 2016 race in his favor and whether Trump sought to obstruct justice when he fired James Comey as FBI director last May.

"When Mueller came across Manafort's illicit activities, he had two choices," Cramer said. "He could ignore that, or he could bring charges."

He added: "These prosecutors have the documents. This was a scathing indictment, and they've got Manafort cold. The question now is whether or not the judge agrees Mueller was authorized to charge him."

When he appointed Mueller last May, Rosenstein gave him broad authority not only to investigate "any links and/or coordination between the Russian government and individuals associated" with the Trump campaign, but also to examine "any matters that arose or may arise directly from the investigation."

He also sent Mueller a memo in August outlining the full scope of his authority. While much of the document was redacted, Rosenstein authorized Mueller to investigate at least two threads related to Manafort: allegations that he colluded with Russian officials during the campaign, and allegations that he committed crimes "arising out of payments he received from the Ukrainian government before and during the tenure of President Viktor Yanukovych."