When the Justice Department charged the Southern Poverty Law Center in April with defrauding its donors, the theory of the case was that the storied civil rights group had secretly funneled money to informants inside white supremacist groups in an effort to support them, not bring them down.

But according to court papers filed on Wednesday, federal prosecutors have turned one of the center’s most active and successful informants into an informant of their own.

The papers, written by the law center’s lawyers, assert that the department capitalized on the informant’s financial problems and exposure to indictment to help him pivot from assisting the center in infiltrating hate groups to helping the government build its criminal case.

Prosecutors are, of course, free to seek information from whomever they want. But the law center’s lawyers said that this informant — known in the papers only as F-9 — went far beyond serving as a normal government witness and came to resemble a member of the government’s investigative team.

While it remains unclear if the government’s lawyers will be able to use F-9’s participation in the case to their advantage, the court papers offer the most detailed portrait yet of a person who has played an outsize role in the prosecution.

The Justice Department did not immediately respond to a request for comment.

F-9 started speaking to investigators as far back as March 2022 in an earlier tax inquiry against the law center that ended without charges. But at least in part at his urging, the court papers said, that investigation was revived last year and retooled to focus on the center’s informant program, which he took part in for decades.