US District Judge Tanya Chutkan in Washington scheduled a hearing for Wednesday.
Eyebrows were raised all the more when, for instance, the detail emerged that an Federal Bureau of Investigation raid on Erickson's South Dakota home yielded a handwritten note that said "How to respond to FSB offer of employment?"
Maria Butina in Moscow.
ABC News first reported that Butina would cooperate with prosecutors.
A spokesman did not respond to a request for comment Monday.
The case against Butina is being prosecuted by the U.S. Attorney's Office in Washington and the National Security Division, and not U.S. Special Counsel Robert Mueller, who is investigating Russian interference in the 2016 U.S. election and any coordination between Moscow and Trump campaign members. However, for several weeks, prosecutors and Butina's lawyer have indicated in court papers that they were negotiating and may have been nearing a plea deal.
Prosecutors say she worked with a Russian official and two United States citizens to try to infiltrate the NRA lobby group and influence Washington's policy towards Moscow.
"We get along with Putin", he told Butina, referring to the Russian president.
Prosecutors in their criminal complaint accused Butina of ingratiating herself with politically powerful Americans and groups, including the NRA, and exploiting those connections to try to advance Russian interests.
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Butina was arrested in Washington on July 15, 2018.
One significant detail is that she has plead guilty to the charge of conspiracy to act as a covert agent of a foreign government, but not to the charge of acting as a covert agent. A judge will determine her sentence but the deal means she could be released from jail and deported to Russian Federation in coming months.
Her lawyers are asking for a sentence of 0 to 6 months, and this is a felony conviction, so she will face deportation.
Butina ran a small Russian group called the Right to Bear Arms that appears to have been funded in part by Russian politician Alexander Torshin, an ally of President Vladimir Putin. She and her lawyer have repeatedly, and unsuccessfully, argued that she should be released on house arrest or at least moved out of solitary confinement.
"The only thing she might agree with is that she should have registered as a foreign agent", he said. Erickson, a US citizen, has reportedly received a "target letter" from prosecutors and may be charged for acting as a foreign agent.
In particular, Butina predicted a Republican likely would win the presidential election and said she could help sway GOP opinion through ties she had built with the NRA. That role gave her access to conservatives intrigued by the idea of the Siberian-born self-made activist agitating for expanded gun rights in Putin's restrictive Russian Federation. He has been subject to sanctions in America, alongside two dozen other senior officials. After it ended, she sent Torshin a message: "We should let them express their gratitude now, we will put pressure on them quietly later".
They identified one of the two Americans mentioned in the criminal complaint as being Paul Erickson, a conservative US political activist who was dating Butina.
Her cooperation will mainly focus on telling investigators about the role of her boyfriend Paul Erickson and her interactions with her Russian handlers.