An attorney for Maria Butina -- the accused Russian agent who was sentenced to 18 months in prison earlier this year -- is alleging “possible misconduct on the part of federal agents or assistant attorneys who investigated and prosecuted Maria,” saying the defense was misled about the role of a well-known, wealthy CEO in the case.

In a letter to the Justice Department obtained by Fox News, Butina’s attorney, Robert Driscoll, revealed Butina’s past relationship with Overstock.com CEO Patrick Byrne. Driscoll claims he told the government he believed Byrne -- “who had a sporadic relationship with Maria over a period of years prior to her arrest” -- was a government informant.

“My speculation was flatly denied,” he said.

MARIA BUTINA SENTENCED TO 18 MONTHS IN PRISON

But Driscoll said he now believes that’s false. “Mr. Byrne has now contacted me and has confirmed that he, indeed, had a 'non-standard arrangement' with the FBI for many years, and that beginning in 2015 through Maria’s arrest, he communicated and assisted government agents with their investigation of Maria.”

Reached via email by Fox News on Friday afternoon, Byrne confirmed the information about his relationship with Butina in Driscoll’s letter.

Byrne’s involvement with Butina was first reported by journalist Sara Carter. In an interview with Carter, Byrne spoke of his decision to come forward with the information, saying: “It was something I knew I had to do. Those running the operation were not honest and, in the end, I realized I was being used in some sort of soft coup.”

The letter is addressed to the DOJ’s Office of Professional Responsibility, as well as Inspector General Michael Horowitz and U.S. Attorney John Durham. Horowitz and Durham are leading investigations related to the Russia probe, and Driscoll said “it appears that some of the issues raised by the information I received relate to FBI counterintelligence operations with respect to Russia leading up to the 2016 election.”

The Justice Department could not be reached for comment Friday afternoon.

The letter said Byrne met Butina for the first time at the FreedomFest convention in Las Vegas in July 2015. Byrne subsequently reported his contact with her to FBI agents, who encouraged him to stay in touch with her, Driscoll wrote.

“As time passed, Byrne became more and more convinced that Maria was what she said she was—an inquisitive student in favor of better U.S.-Russian relations—and not an agent of the Russian government or someone involved in espionage or illegal activities,” the letter states. “He states he conveyed these thoughts and the corroborating facts and observations about Maria to the government.”

In April, Butina was sentenced to 18 months in prison by a federal judge in Washington after pleading guilty last year to a conspiracy charge. . She will be removed from the U.S. promptly on completion of her time, the judge added, and returned to Russia.

An emotional and apologetic Butina said in court at her sentencing she was “truly sorry” and regrets not registering as a foreign agent.

“I feel ashamed and embarrassed,” she said, adding that her “reputation is ruined.”

Butina has been jailed since her arrest in July 2018.

Prosecutors had claimed Butina used her contacts with the National Rifle Association and the National Prayer Breakfast to develop relationships with U.S. politicians and gather information for Russia.

In their filings, prosecutors claimed federal agents found Butina had contact information for people suspected of being employed by Russia's Federal Security Services, or FSB, the successor intelligence agency to the KGB. Inside her home, they found notes referring to a potential job offer from the FSB, according to the documents.

Investigators recovered several emails and Twitter direct message conversations in which Butina referred to the need to keep her work secret and, in one instance, said it should be "incognito." Prosecutors said Butina had contact with Russian intelligence officials and that the FBI photographed her dining with a diplomat suspected of being a Russian intelligence agent.

In his letter, Driscoll said Byrne, as a CEO of a public company, “is a credible source of information” and has “little to gain but much to lose by disclosing a sporadic relationship with Maria.”

"He also told me that some of the details he provided the government regarding Maria in response was exculpatory—that is, he reported to the government that Maria’s behavior and interaction with him was inconsistent with her being a foreign agent and more likely an idealist and age-appropriate peace activist,” Driscoll said.

He added: “His claims are worthy of investigation. Indeed, he has much to say about the government’s handling of Maria’s case that go far beyond the…issue I raise in this letter.”

The Associated Press contributed to this report.