Peter Strzok was "at the heart" of the investigation surrounding Russian involvement with the Trump campaign, former U.S. Attorney Brett Tolman explained Friday.

In an interview on "Fox & Friends," Tolman said that the FBI's case against former Trump national security adviser Michael Flynn was tainted by the actions of former agent Strzok.

"Well, first of all [Flynn] is entitled to talk to an ambassador," Tolman said, referring to a conversation Flynn had with Russian ambassador Sergey Kislyak during the president's transition period. During Robert Mueller's Russian investigation, the FBI questioned Flynn on their meeting and newly-released documents indicated that FBI agents planned to get Flynn “to admit to breaking the Logan Act” when speaking with Kislyak.

STRZOK STOPPED FBI FROM ENDING FLYNN PROBE DESPITE LACK OF 'DEROGATORY' EVIDENCE, UNSEALED DOCUMENTS REVEAL

"There is not an underlying crime there. There is no evidence that they are really concerned about the behavior of your national security advisor," he argued. "Instead, what they have is their own doubt that there is anything that's gone on that's wrong [and] their own effort to try to create something when they don't have it."

FBI Deputy Assistant Director Peter Strzok is seated to testify before the House Committees on the Judiciary and Oversight and Government Reform during a hearing on "Oversight of FBI and DOJ Actions Surrounding the 2016 Election," on Capitol Hill, Thursday, July 12, 2018, in Washington. (AP Photo/Evan Vucci)

"Then you see someone like Peter Strzok keeping the investigation open. For a former federal prosecutor like myself or anybody else, it should be offensive because you have to be just as good at getting rid of a case as you area t driving forward and presenting a case for indictment," Tolman added.

Internal FBI documents unsealed Thursday indicate that Strzok ordered the investigation of Flynn to remain open even after it was slated to be closed due to a lack of so-called "derogatory" information.

The materials surfaced just a day after explosive FBI communications revealed that top bureau officials discussed their motivations for interviewing Flynn in the White House on January 24, 2017 -- and openly questioned if their "goal" was "to get him to lie, so we can prosecute him or get him fired."

"Look, "they had an agenda against this president. Don't forget that," Tolman pointed out.

"They had to have high-level prosecutions. There are over 300,000 federal statutes that have criminal penalties. I used to tell people: 'Give me anybody. Let the FBI spend some time with them. You could find a criminal charge to bring against them,'" he told the "Friends" hosts.

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"Here, this is an area where it's completely subjective. The agents themselves get to make the determination whether or not they think there is honesty or not. They indicated -- the notes suggest -- they didn't think he was being dishonest, but Peter Strzok, who is at the heart of this storm that was unleashed on a sitting president in his administration, he is at the heart of it and he keeps it alive," he concluded.

"They had to have [a check] in order to justify what they were doing with the Mueller investigation."

Fox News' Gregg Re, David Spunt, and Wilson Miller contributed to this report.