Christine Beatty, who served as Kilpatrick's chief of staff, had an affair with him while he was mayor and was accused with him of perjury. She appeared before the grand jury Wednesday, The Detroit News reported.
Neither Beatty nor her lawyer, Jeffrey Morganroth, would comment on what she said.
"I can confirm she was subpoenaed to appear, not as a target or as a subject, but as a witness," Morganroth said. "She answered all their questions and cooperated fully."
Beatty's appearance suggests a federal investigation of Kilpatrick's administration is still being conducted. The mayor has already been indicted on federal corruption charges.