CINCINNATI (Reuters) - The Web site of the Ohio state agency that handles voter registration and other election information was shut down briefly after it was hacked, an official said on Tuesday, vowing to guard against fraud in the key battleground state in the November 4 presidential contest.

Ohio Secretary of State Jennifer Brunner said the agency temporarily took the Secretary of State Web site at www.sos.state.oh.us down on Monday after “one or more” security breaches were detected.

The site was restored to partial service on Tuesday after technicians worked overnight to ensure that no information could be compromised.

“Our focus is and has always been to protect the vote of every eligible Ohio voter from any kind of fraud, be it voter registration fraud, illegal voting or vote suppression. This action has been taken to detect and prosecute any illegal breach of our voting infrastructure to maintain voter confidence,” Brunner said.

Ohio Gov. Ted Strickland, a Democrat, said in a separate statement that Brunner and her family had been subjected to “repeated serious threats.”

With 20 of the 270 Electoral College votes needed to win the presidential election, Ohio is a central front in this year’s White House campaign between Republican John McCain and Democrat Barack Obama. Ohio was the state that secured Republican President George W. Bush his 2004 re-election victory.

The Ohio State Highway Patrol is assisting with an investigation into the security breach, Brunner said.

There were widespread complaints of voting problems in Ohio in the 2004 presidential election, and Republicans and Democrats in the state have been battling in recent weeks over voting registration rules and early voting procedures.

Brunner, a Democrat, said her office has also been assaulted in recent weeks by a barrage of phone calls and e-mails containing “menacing messages and even threats of harm or death.” Last week, a suspicious package covered with threatening messages and containing an unidentified powder was mailed to the office, Brunner said.