COLUMBUS, Ohio (CNN) -- The Web site of the Ohio secretary of state was back up and running Tuesday, a day after officials pulled the site partially offline when it was hacked by unknown outsiders.

Ohio Secretary of State Jennifer Brunner says her office has received death threats.

Secretary of State Jennifer Brunner told CNN that "fortunately" no sensitive material was breached in the incident.

"We found it in time that we were able to mitigate any problems," she said.

However, Brunner said it wasn't the first time her office, which has been embroiled in an ongoing dispute with the state Republican Party over alleged voter registration fraud, has been targeted and that death threats have been part of the package.

"There's been a concerted effort to barrage the office with e-mails and phone calls," she said, "not just to our elections division but to business services division. My employees have been real troopers here.

"The First Amendment is alive and well in the United States, and you know that, and they're not coming just from Ohio. A lot of times, they're coming from other states. But this is all part of the process. And my employees are doing a fantastic job of staying focused on what they need to do."

Brunner said the Ohio Highway Patrol is investigating the hack, the calls and e-mails, as well as a suspicious package with a message that said "Death to Obama supporters" and an unidentified powder. That package, addressed to an old office building, was delivered by her office by mail last week, she said.

The secretary attributed the troubles to the "contentious nature" of the litigation begun by the Ohio Republican Party.

Last week, the U.S. Supreme Court blocked a lower court order directing Brunner -- a Democrat -- to update the state's voter registration database after information provided by some newly registered voters did not match up with Social Security and driver registration numbers.

The state Republican Party had asked for enforcement of a temporary restraining order, but the justices, in an unsigned order, denied that request.

Brunner and other elections officials had appealed an earlier ruling from the 6th U.S. Circuit Court of Appeals in Cincinnati siding with the state GOP.

The state Republican Party contends that there is widespread voter fraud in Ohio, considered by many to be a crucial battleground state for the 2008 presidential election. Officials also allege Brunner "turned off" its process for verifying voter registrations while allowing Ohioans to cast ballots on the same day they registered.

State GOP Chairman Bob Bennett has accused Brunner of concealing fraudulent voter registrations in hopes of swinging the state to Sen. Barack Obama, the Democratic presidential candidate.

Voter registration fraud involves submitting registration forms for people not eligible to vote, and rarely turns into the more serious crime of voter fraud, in which people knowingly vote illegally in an attempt to defraud the election system.

Most voter registration fraud cases involve registrars, hired by non-profit organizations to register people to vote, who submit forms they filled out themselves rather than collected from actual potential voters, according to the Brennan Center for Justice.

Cases of actual voter fraud are themselves extremely rare, the center said.

Republican Party officials are challenging voter registration in several swing states in this election cycle, prompting charges from some Democrats that the GOP is attempting to suppress the Democratic vote.

Similar charges were leveled in 2004 when George W. Bush narrowly defeated John Kerry in Ohio to secure a second term, but the Department of Justice failed to uncover any evidence of intentional vote suppression.

CNN's Mary Snow contributed to this report.

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