(CNN) Special counsel Robert Mueller's team last year made clear it wanted former Trump campaign deputy Rick Gates' help, not so much against his former business partner Paul Manafort, but with its central mission: investigating the Trump campaign's contact with the Russians. New information disclosed in court filings and to CNN this week begin to show how they're getting it.

In a court filing earlier this week, the public saw the first signs of how the Mueller team plans to use information from Gates to tie Manafort, the former Trump campaign chairman, directly to a Russian intelligence agency. Mueller's team alleges that Gates was in contact with a close colleague of Manafort's who worked for a Russian intelligence agency -- and that Gates knew of the spy service ties in September and October 2016, while he worked on the Trump campaign. Gates would have to talk about the communication with the man if prosecutors wanted, according to his plea deal.

That's in line with what prosecutors told Gates months ago during high-stakes negotiations, CNN has learned. They told him they didn't need his cooperation against Manafort, according to a person familiar with the investigation, and instead wanted to hear what he knew about contact between the Trump campaign and Russians.

The extent of Gates' knowledge about any such contact or what he told prosecutors hasn't been made public.

As part of Gates' agreement to cooperate with the special counsel a month ago, he earned a vastly reduced potential sentence and had several charges dropped in two criminal cases against him.

Gates' plea also adds to mounting pressure on his co-defendant Manafort -- who so far the government is making a central player in the investigation -- to change his plea and potentially help investigators. Under his plea agreement, Gates still could be called to testify against Manafort.

Mueller's court filing Tuesday night, in a separate case for a lawyer whose firm did legal work for Gates and Manafort, made public the most direct effort yet by Mueller's team to draw a line between Manafort and the Trump campaign to Russian operatives. Prosecutors called the details of Gates' contact with the Russian intelligence officer during the campaign "pertinent to the investigation."

Russian contact

The alleged Russian intelligence agent, referred to as "Person A" in the court filing, appears to be Konstantin Kilimnik, a former employee who worked with Manafort's firm and lived in Kiev and Moscow, according to sources familiar with the investigation. In December, the Mueller prosecutors made a similar unnamed reference to Kilimnik, saying he is "assessed to have ties to a Russian intelligence service." That was related to a Kiev newspaper op-ed that Kilimnik helped edit in consultation with Manafort late last year, which prosecutors said could violate a court-imposed gag order on Manafort.

The criminal allegations facing Gates and Manafort encompass work they did in the years prior to the 2016 election. Manafort has pleaded not guilty in two federal criminal cases stemming from Mueller's prosecution, one in Washington, DC, and another one in Virginia.

A chief criticism from President Donald Trump and his defenders has been that the charges brought so far by the special counsel don't relate directly to Mueller's central mission investigating possible illegal coordination between the Trump campaign and the Russian government, which the President and others often shorthand using the term collusion. The accusation related to Kilimnik and ties to Russian intelligence, made in Mueller's Tuesday filing, begins to answer that criticism, but so far without actually making the charge in Manafort's case.

A lawyer for Manafort declined to comment. Kilimnik didn't comment. Last year, Kilimnik told The Washington Post he has "no relation to the Russian or any other intelligence service."

Gates' current lawyer also declined to comment for this story.

What Gates knows

Gates may have information of value to prosecutors beyond his business dealings with Manafort, according to sources familiar with his role. He never grew close to Trump, but he had ties with other members of Trump's inner circle, including Manafort and Tom Barrack, a fundraiser and close friend of Trump's. He also developed a reputation for keeping tabs on what others were up to, one source said.

Gates worked alongside Manafort during the critical summer of 2016 when senior campaign officials, including Manafort, met at Trump Tower in New York with a group of Russians who had promised damaging information on Democratic nominee Hillary Clinton.

"Was he in the strategy meetings? No. But he was an implementer," one person said of Gates. So while he may not have participated in the Trump Tower meeting with the Russians, he may still have knowledge of the meeting or whether those Russians were ever introduced to Trump himself.

"He would be the kind of person who would probably know that," this person said.

Even after Manafort was fired from the campaign, Gates stuck around. He then went on to work with Barrack, while Barrack ran the presidential inaugural committee following the election.

Legal team troubles

Before the indictment, prosecutors had told Gates he faced the risk of criminal charges -- the beginning of a strategy to pressure him, according to two sources.

Gates' various legal teams -- and there have been many -- kept in touch with Mueller's office throughout the investigation.

Once prosecutors made a plea offer following Gates' indictment, that offer never changed: He could plead guilty to a conspiracy count if he helped the investigation.

In all, Gates held out for almost four months. He initially pleaded not guilty with the help of a public defender after he split with his previous attorney.

He then hired three lawyers, who each worked in their own small firms, to take him to trial.

But the financial stress on Gates mounted. He failed to pay some of his legal bills.

"The reality of how long this legal process will likely take, the cost, and the circus-like atmosphere of an anticipated trial are too much," Gates told friends and family in an email the morning he pleaded guilty.

Despite the risk for Manafort and the cost, Gates engaged a third set of private attorneys, led by Thomas Green of the large law firm Sidley Austin, to negotiate his plea deal with Mueller in January and February.

Even as Green finalized Gates' plea throughout February, Gates wavered on his willingness to cooperate with the prosecutors. For months, he held on-and-off conversations with a fourth option for a private defense attorney to take him to trial: Barry Pollack. Pollack had previously worked closely with Manafort's attorneys.

At least three other well-known criminal defense law firms tried to help Gates over the past several months, according to multiple sources. One law partner made calls to find him lawyers, another two lawyers reviewed the financial allegations, and a third firm offered to get involved. Manafort's attorneys were engaged in some of the efforts but not all.

More recently, outside lawyers created a legal defense fund to help defendants in the Mueller investigation fight their charges.

Yet it wasn't enough. Gates never hired Pollack nor any other attorney to take him to trial after he was released from house arrest in early January.

Prosecutors revealed after one of his interviews with the Mueller team, on February 1, that they believed he had lied to them about a detail of Manafort's lobbying efforts. The lie resulted in a second charge added to his possible plea. Gates chose to cooperate and plead guilty eight days later.

CORRECTION: This story has been updated to correctly state that a legal defense fund to help defendants in the Mueller investigation was created by outside lawyers, not the Republican National Committee.