Updated at 5:24 p.m. ET
Opponents of special counsel Robert Mueller ramped up their attacks over the weekend with a new claim that he improperly collected thousands of emails from President Trump's transition team and is using them as an illegitimate basis for much of his investigation.
Mueller's office said his team has obtained all the evidence it's using in its investigation properly. And Deputy Attorney General Rod Rosenstein, who appointed him, told Congress last week that he monitors Mueller's operation closely and has seen nothing improper.
This weekend's dispute is the latest chapter in an ongoing campaign by Trump's Republican allies against Mueller and his team.
Upon his return to the White House from an overnight trip to Camp David, the president weighed in on the controversy surrounding his transition team's emails, telling reporters, "It's not looking good."
"It's quite sad to see. My people were very upset about it. I can't imagine there's anything on [the emails] quite frankly, because as we said there's no collusion, no collusion whatsoever."
He also told reporters he has no plans to fire Mueller.
The anti-Mueller camp raised the stakes on Saturday by charging that Mueller had improperly obtained tens of thousands of emails from as many as a dozen accounts associated with the Trump transition, and investigators had been using information they contained in their investigation.
At issue is how the special counsel obtained the materials.
The emails were stored with General Services Administration, a government agency that does a range of things like providing office space for federal employees. It also hosted the incoming administration's email system, which ended in "@ptt.gov" — or presidential transition team.
The special counsel reportedly got the email trove from the GSA, not from Trump For America, a still-functioning arm of Trump's transition team.
The special counsel's office pushed back on the characterizations that the emails were obtained unlawfully. In a statement Sunday, spokesperson Peter Carr said:
"When we have obtained emails in the course of our ongoing criminal investigation, we have secured either the account owner's consent or appropriate criminal process."
But a lawyer for the Trump transition on Saturday sent a letter to two congressional committees arguing that the materials were acquired in a way that was "unlawful."
According to Axios, which broke the story Saturday, the transition emails contain "sensitive exchanges" on topics ranging from policy planning, potential appointments and views on senators who would be involved with confirming political appointees.
Also on Saturday, Kory Langhofer, a general counsel for Trump For America, sent a letter to chairs of the Senate Committee on Homeland Security and Governmental Affairs and the House Committee on Oversight and Government Reform.
In the letter, obtained by Politico, Langhofer calls the emails "private, privileged materials," not government property, and informs the committees of what he described as "unlawful conduct that undermines the Presidential Transition Act of 1963."
Langhofer went on to say:
"The Special Counsel's Office has extensively used the materials in question, including portions that are susceptible to claims of privilege, and without notifying TFA or taking customary precautions to protect TFA's rights and privileges."
Langhofer also asserts in the letter sent to the chairs of the congressional committees that "TFA owned and controlled the PTT emails and data" adding that "GSA had no right to access or control the records but was simply serving as TFA's records custodian."
A lawyer for TFA, who spoke on background because he was not authorized to speak to media, told NPR it found out on Dec. 13 that GSA officials turned over the emails to the special counsel in September.
The lawyer added that TFA had a "reasonable expectation of privacy in those emails" and claims that in order for the special counsel to have received the materials it would have needed a warrant or a subpoena.
Complicating matters is that Langhofer said in the memo that there was an understanding that TFA was the custodian of the emails. He said the agreement was laid out in June before a lawyer at GSA, Richard Beckler, a Trump appointee, who has since died.
This dispute over the lawful obtaining of this batch of emails is the latest in a series of ongoing attacks, particularly from conservatives, on the Department of Justice investigation looking into what role Russia played in interfering in the 2016 presidential election.
As NPR's Carrie Johnson points out:
"GOP voices are accusing the team assembled by special counsel Robert Mueller of bias against President Trump — and they're appearing to set the stage for some action. Senior Justice Department officials are defending the investigation, which has already secured indictments or guilty pleas against four people with ties to the Trump campaign."
There are rumors around Washington suggesting Trump's firing of Mueller is imminent. But White House special counsel Ty Cobb told NPR's Tamara Keith that's not the case. He said:
"As the WH has persistently said for months, there is no consideration being given to firing the Special Counsel with whom we continue to cooperate with the expectation of an appropriate and timely result."
During a House Judiciary Committee hearing last week, Republicans slammed the special counsel's office over what they said was the Democratic-leaning bias of some of its key lawyers and investigators.
At that hearing, Rosenstein, the Justice Department official who oversees Mueller's investigation, told lawmakers he is aware of how Mueller is carrying out the probe and he saw no reason to terminate him.
"I know what he's doing," Rosenstein said. "If I felt he was doing something inappropriate, I would take action."
As it stands, Mueller's team has secured indictments against Trump's former campaign chairman Paul Manafort and Manafort's business associate Rick Gates. Mueller's team has also gotten guilty pleas from Trump's first national security adviser and campaign adviser Michael Flynn and George Papdopolous, a foreign policy adviser to Trump during the campaign. Both men pleaded guilty to lying to FBI agents.