The family of a federal judge had just been brutally murdered. And as Harold C. Turner remembered it, it seemed an opportune time for the F.B.I. to make use of his secret role as a confidential informant.

Sitting together at a New Jersey diner, the F.B.I. agents told Mr. Turner, an Internet radio host and provocateur, that they wanted his help identifying a killer, whom they believed to be a member of a white supremacist organization, Mr. Turner testified on Wednesday in United States District Court in Brooklyn.

Mr. Turner enjoyed a devoted following among such groups because of the racist and inflammatory views he espoused on his program. The F.B.I.’s request, Mr. Turner said, came with a suggestion: “Ratchet up the rhetoric.”

Mr. Turner said he immediately obliged. That afternoon he posted a picture of the judge, Joan Humphrey Lefkow of Federal District Court in Chicago, on his blog below the headline “Gotcha!” He later appeared on television twice to discuss the case — for which he said he was paid by the F.B.I. — both times declaring Judge Lefkow “worthy of death.”