OREGON CITY -- A Clackamas County grand jury on Wednesday indicted

, 55, on charges of ballot tampering.

left blank to cast additional votes for Republicans.

Swenson, a Beavercreek resident, is charged with two counts each of unlawfully altering a cast ballot, unlawfully voting more than once and first-degree official misconduct.

State investigators said they believe Swenson acted alone and found no evidence others were involved.

Jeff Manning, spokesman for the state Department of Justice, declined to specify the number of ballots Swenson is suspected of altering.

Authorities previously said on Nov. 6 -- Election Day -- that six suspect ballots had been identified.

"As long as a trial is pending, we're not going to be able to go into detail, I'm afraid," Manning wrote in an e-mail.

Tampering with ballots and voting more than once are Class C felonies punishable by up to 5 years in prison and a $125,000 fine. Official misconduct is a Class A misdemeanor, punishable by up to one year in a jail and a $6,250 fine.

"The integrity of Oregon elections is of paramount importance," Attorney General Ellen Rosenblum said in a press release. "The Department of Justice has made this investigation a high priority since the allegations came to light."

Oregon Department of Justice officials launched their investigation after a co-worker reported seeing Swenson altering ballots Oct. 31. The Clackamas County Elections Office reported the incident that afternoon to the Oregon Secretary of State's Office, which referred the case to the Department of Justice. The Secretary of State's Office handles elections complaints but does not investigate felony matters.

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Swenson was immediately relieved of her duties. Swenson had worked an unknown number of elections since at least 2010, Scot Sideras, assistant county counsel, previously said. It's unclear whether the investigation of Swenson extends to previous elections.

, frustrating voters who feared their ballots would not be counted.

"if the original voter intent can be established."

"We have zero tolerance for voter fraud in Oregon," Brown said in a press release this morning. "Security procedures are in place to detect this kind of conduct. The system worked, and no person's ballot was impacted. Oregonians can be confident in their election results."

Brown added that the annual security plan county elections officials submitted to her office was followed.

County ballots featured two highly contested races for county chair and Position 4, won by Republican challengers John Ludlow and Tootie Smith, respectively. Current Chairwoman Charlotte Lehan and Commissioner Jamie Damon, both Democrats, declined to formally concede citing the ongoing state investigation.

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