ST. LOUIS • The city’s top prosecutor said her office “uncovered important evidence” in a voter fraud investigation dealing with an August primary statehouse race that has since been reversed by voters.

The case — which centered on the validity of absentee ballots — has now been turned over to the U.S. attorney’s office.

In an email with the subject line “Fraud,” a spokeswoman for Circuit Attorney Jennifer M. Joyce said Tuesday that based on prosecutors’ findings, “the U.S. attorney’s office has agreed to expand the investigation at Joyce’s request.”

U.S. Attorney Richard Callahan confirmed his office had been monitoring the investigation since it began in late August.

“But in recent weeks, they asked us if we would assume responsibility for the main role, and we agreed,” Callahan said. Callahan declined to offer any specifics about when the investigation might conclude.

“I’m sensitive to timeliness, and we will do it as quickly and as thoroughly as we can,” he said. A spokesman for Joyce’s office said she could not provide any more information because it’s a pending investigation.