Updated

Federal officials confirm that a vote fraud investigation is unfolding on Indian reservations in South Dakota, home of one of the tightest U.S. Senate races in the nation.

Federal officials in Washington told Fox News that so far, the alleged fraud is said to have occurred on the Cheyenne River Reservation and the Pine River Reservation, and an investigation has been ongoing in six counties, including Dewey, Ziebach and Fall River.

According to officials, the FBI has uncovered the registration of minors, dead people, and people who do not exist. Many of the registrations have included bogus names and invalid addresses.

Investigators said in one case a woman was registered to vote a week after her death.

They have also found multiple absentee ballots distributed to the same registered voter but returned with different signatures, the officials said.

The case was brought to the attention of the South Dakota attorney general's office when county auditors began discovering problems with absentee ballot requests and votes. State Attorney General Mark Barnett said the investigation has been ongoing for two weeks.

Barnett said that he hoped invalid absentee ballots haven't been filed. Absentee voting began Sept. 24 and the registration deadline is Oct. 21.

"I don't even want to think about it," Barnett said. "A lot of absentee ballots are going to get looked at."

Federal sources said the key suspect in the investigation is a former staffer of the state Democratic Party, whom is alleged to have falsified voter forms. The party itself has not been implicated. Officials said that because of the size of the alleged fraud, they expect to find accomplices.

Bret Healy, executive director of the state Democratic Party, said the worker was fired as soon as the party learned of the allegations. Healy said party officials notified the U.S. attorney.

South Dakota does not require a photo ID to register to vote and absentee ballots can be obtained without appearing personally.

Last week, Attorney General John Ashcroft attended a symposium in Washington, D.C., to promote legal voting this year. The national Democratic Party also said that it would monitor elections this year to make sure that those who want to vote are given access to the polls.

On Thursday night, the House gave final approval to an election reform bill that provides $3.8 billion to states to update equipment. The Senate voted 92-2 for the reforms on Wednesday. President Bush has said he would sign the legislation.

Republicans had insisted that measures also be included to deter fraud, including a provision that those who register by mail bring some form of identification to their polling places.

That earned scorn from some lawmakers who said that the provision would erect barriers to voting.

The South Dakota U.S. Senate race between incumbent Democrat Tim Johnson and Republican Rep. John Thune is one of the hottest in the country.

The race is among the eight tightest in the country which are likely to determine which party controls the Senate majority next year. The latest polls indicate Thune is ahead 48-43.

Thune has said he wouldn't rule out a trip to court if there's evidence of widespread voter registration fraud and he loses by a close margin.

"This race is close and both sides have to be prepared in a race this close. The Republican Party, hopefully, has taken steps to deal with issues in that respect," Thune told the Sioux Falls Argus Leader newspaper.

Campaign spokeswoman Christine Iverson said Tuesday that the Thune camp has "no plans to contest this election, and we certainly hope it doesn't come to that."

Because South Dakota is home state to Senate Majority Leader Tom Daschle and because President Bush recruited Thune to run, this contest is seen as a proxy fight between Daschle and Bush.

Fox News' Carl Cameron and The Associated Press contributed to this report.