Washington(CNN) Former Trump campaign aide Rick Gates has quietly added a prominent white-collar attorney, Tom Green, to his defense team, signaling that Gates' approach to his not-guilty plea could be changing behind the scenes.

Green, a well-known Washington defense lawyer, was seen at special counsel Robert Mueller's office twice last week. CNN is told by a source familiar with the matter that Green has joined Gates' team.

Green isn't listed in the court record as a lawyer in the case and works for a large law firm separate from Gates' primary lawyers.

Green's involvement suggests that there is an ongoing negotiation between the defendant's team and the prosecutors. At this stage, with Gates' charges filed and bail set, talks could concern the charges and Gates' plea. The defense and prosecution are currently working together on discovery of evidence.

Gates pleaded not guilty in October to eight charges of money laundering and failing to register foreign lobbying and other business. His longtime business partner, former Trump campaign chair Paul Manafort, pleaded not guilty to nine counts in the same case as Gates.

For months, court-watchers -- including Gates' own attorneys -- have anticipated additional charges against the defendants. Superseding indictments, which would add or replace charges against both Gates and Manafort, have been prepared, according to a source close to the investigation. No additional charges have been filed so far. When there is a delay in filing charges after they've been prepared, it can indicate that negotiations of some nature are ongoing.

A few other developments in the Manafort and Gates case point to a softer touch that Mueller's team is using with Gates. Manafort, like Gates, has pleaded not guilty in the case, yet prosecutors and his defense attorneys have squabbled throughout Manafort's recent court action, more so than Gates' lawyers. Manafort's and Gates' lawyers work separately and do not use a joint defense agreement, which leaves open the possibility that at some point the defendants could have diverging interests.

The judge overseeing the case finished a drawn-out process of changing Gates' bail condition last week. The federal prosecutors didn't stand in the way of Gates being released from house arrest, even after they and the judge found that his assets could barely back his $5 million bail.

At a hearing last week, the judge also acknowledged that the deadlines for legal work before a trial could be different for Gates and Manafort.

"We are the least prepared of anyone here and we want to do a good job and we need that time to be able to do it," Gates' lawyer Walter Mack told the judge as they discussed filing deadlines. A schedule hasn't been finalized.

Should a deal be worked out, it would mean Mueller has the cooperation of another Trump campaign insider.

Gates and Manafort were business partners for about a decade before Manafort brought him onto the Trump campaign as his deputy. Their roles on the campaign grew as they were given more responsibilities and when campaign manager Corey Lewandowski resigned in June 2016.

The charging documents against Trump campaign adviser George Papadopoulos revealed that Papadopoulos emailed Manafort about his contacts with government-connected Russians and that the Russians were "open for cooperation" and wanted an opportunity to meet Donald Trump. Manafort forwarded the message to Gates and said, "Let's discuss. We need someone to communicate that (Trump) is not doing these trips. It should be someone low level in the campaign so as not to send any signal."

Manafort and Gates were also in charge during the 2016 Republican National Convention in Cleveland, where a handful of Trump campaign advisers met with then-Russian ambassador Sergey Kislyak and a group of campaign aides controversially changed the Republican Party platform regarding Ukraine.

Gates remained on the campaign after Manafort's hasty resignation in August 2016, but his role was diminished and he stopped working out of the campaign headquarters in Trump Tower. Manafort resigned in the wake of damaging stories about his lobbying in Ukraine -- the same work that got him and Gates indicted.

Gates later played a key role on Trump's Presidential Inaugural Committee. His interactions with Trump and other prominent campaign officials could be of interest to Mueller's prosecutors.

Green is one of the biggest names in Washington's white-collar defense world. He is known for his willingness to fight a case to trial. Yet he's also well-versed in cutting plea deals -- like the outcome he negotiated in 2015 for his client Dennis Hastert, the former House speaker and Illinois congressman.

"If I were in trouble, I'd call out to somebody like Tom," said Carol Bruce, a DC white-collar lawyer who served as special counsel in a US Senate ethics investigation and as independent counsel in another federal investigation.

Green declined to comment and declined to say he represented Gates. So did Gates' primary lawyers, Shanlon Wu in Washington and Mack in New York, citing the judge's gag order on their case. A spokesperson for Mueller's office declined to comment.