Paul Erickson, a longtime Republican politico whose Russian girlfriend is in jail on charges she acted as a covert foreign agent, has been informed that he may face similar accusations. The Daily Beast reviewed a “target letter” that federal investigators sent Erickson’s lawyer, which said they are considering bringing charges against him under Section 951 of the U.S. code—the law barring people from secretly acting as agents of foreign governments.

The letter also said the government may bring a conspiracy charge against Erickson, who is the boyfriend of accused foreign agent Maria Butina. The letter, which was sent in September by investigators working out of the U.S. attorney’s office in Washington, does not accuse Erickson of any crimes or guarantee that he will face charges.

If prosecutors bring the charges named in the letter, Erickson would be the first American embroiled in the 2016 Russia investigation charged under a statute that Justice Department lawyers describe as “espionage-lite.”

“Charging an American under 951 in the context of the Russia investigation is especially serious because that statute is generally reserved for espionage-like cases, such as intelligence-gathering on behalf of a foreign government,” said Ryan Goodman, a former Defense Department attorney who now teaches at the New York University School of Law.

“Essentially what it would say is that an American was acting to advance the interests of a foreign power, contrary to the interests of the United States of America,” said Renato Mariotti, a former federal prosecutor.

A person familiar with the investigation told The Daily Beast that federal law-enforcement officers have interviewed people in Erickson’s orbit, some of whom provided statements to the FBI. Law-enforcement officials asked those sources about the former political insider’s business dealings and his reputation in conservative political circles, according to that person. As The Daily Beast previously reported, several of Erickson’s former business partners have claimed he defrauded them. The U.S. attorney’s office in South Dakota is leading an investigation into those claims.

Justice Department investigators aren’t the only ones interested in Erickson. Staffers with the Senate intelligence committee, which is probing Russian meddling in the 2016 presidential elections, have also asked to speak with him. But a lawyer for Erickson told them he would plead the Fifth Amendment if subpoenaed to testify, a source familiar with those communications confirmed to The Daily Beast. Investigators will not force Erickson to appear just to take the Fifth. William Hurd, who is representing Erickson on the Senate intelligence committee matter, declined to comment.

According to The New York Times, Erickson wrote an email to the Trump campaign in May 2016 offering to set up a back-channel meeting between the candidate and Russian President Vladimir Putin.

“Putin is deadly serious about building a good relationship with Mr. Trump,” Erickson wrote, the Times reported. “He wants to extend an invitation to Mr. Trump to visit him in the Kremlin before the election.”

By then, Erickson had known Butina for years. Butina, a gun-rights activist from Russia, attended grad school at American University while building relationships in the U.S. conservative movement, with Erickson often serving as her guide.

The pair made no secret of their affinity for Russia. As The Daily Beast previously reported, at her birthday party, she dressed as a Russian empress and he dressed as Rasputin. Guests drank vodka from a bottle emblazoned with a hammer and sickle.

And as Butina built relationships with conservative leaders, Erickson didn’t exactly keep his role under wraps. According to court documents filed by the prosecutors charging Butina, someone called “Person 1” and widely believed to be Erickson boasted that he was involved in “securing a VERY private line of communication between the Kremlin” and the Republican Party.

The two also worked together on various other projects, including a deal to secure Russian jet fuel for an American middleman. Erickson and Butina worked on the failed business venture with Donna Keene, the wife of former NRA President David Keene, in 2017.

William Hurd of Troutman Sanders, a lawyer for Erickson, declined to comment. A spokesperson for the U.S. attorney’s office in D.C. declined to comment as well.

The U.S. Attorney’s Manual recommends that prosecutors send target letters to alert people they are in the feds’ sights.

“At the least, it’s a preliminary determination that they’re going to proceed to indictment,” said Sol Wisenberg, the co-chairman of the white-collar practice at Nelson Mullins.

Wisenberg said that while people can generally expect to be indicted within a few months of receiving a target letter, there’s no firm rule on how quickly any indictment should follow, if at all.

If investigators charge Erickson under Section 951, he could be the first American publicly accused of acting (or trying to act) as an agent of a foreign government in connection with Russia’s 2016 interference. Other Americans roped up in Russia investigations—including former Trump campaign chief Paul Manafort and lobbyist Sam Patten—have faced charges for illegal lobbying, in violation of the Foreign Agents Registration Act. But 951 is different. It’s a rare charge, and the Justice Department’s inspector general wrote in 2016 that prosecutors in its elite National Security Division describe it as “espionage-lite.”

“[A] Section 951 case generally involves espionage-like or clandestine behavior or an otherwise provable connection to an intelligence service, or information-gathering or procurement-type activity on behalf of a foreign government,” the inspector general wrote.

Prosecutors in the D.C. U.S. attorney’s office charged Butina with violating Section 951.

Butina said on several occasions that she helped facilitate communications between the Trump campaign and Russia, multiple sources told The Daily Beast for a February 2017 story. In July, the D.C. U.S. attorney’s office charged her with acting as a covert foreign agent and conspiring to commit a crime. Both those charges are also identified in the Erickson target letter.

Special counsel Robert Mueller is not handling Butina’s prosecution. A spokesperson for his office declined to comment on why that is.

Butina is currently in an Alexandria, Virginia, jail. Erickson visits her there regularly, two individuals with knowledge of the meetings told The Daily Beast.