The tape, sold as a bootleg on the streets of New York, Los Angeles and Chicago, was notorious: It showed the R&B singer R. Kelly having sex with and urinating on a girl who prosecutors said was barely a teenager.

But although the tape was shown at Mr. Kelly’s ensuing child pornography trial in 2008, the girl and her immediate family refused to testify, a choice that was seen as crucial to the decision to acquit him.

In a sign of how dramatically Mr. Kelly’s fortunes have changed, a lawyer for that girl, now a woman in her 30s, said on Tuesday that she was cooperating with federal investigators.

The extent of her cooperation was not immediately clear, but the statement from her lawyer, Christopher L. Brown, came just days after Mr. Kelly was accused in a federal indictment of paying the girl and her father to stay quiet, and in some cases to lie to investigators to protect him.