NAPOLEON, Ohio — Officials have sent letters to more than 17,000 Henry County voters informing them that personal information about them might have been exposed in a computer hack, Henry County Commission President Glenn Miller said today.

Mr. Miller said the hack was discovered when a window popped up randomly on an employee’s computer. The incident led officials to discover that someone had once opened a file in the system that contained voter information, but authorities do not believe information was copied from the file.

Affected voters were sent a letter last Thursday telling them hackers had gained access to the database. County officials also offered a free year of service from a credit-monitoring company, but said it does not appear any data was stolen.

The letter says the county was targeted by ”ransomware” with the intent of forcing the county to pay to unlock its own data. However, the letter from the Henry County commissioners said no ransom was paid and the data was recovered from backup files. Mr. Miller said it is believed no information was taken out of the database.

“There would have been some indication of a movement of data and there was no indication,” Mr. Miller said.

The attack occurred Oct. 31.

He said there were 17,841 records in the file from the county’s voter registration list. With each name was the person’s address, phone number, ZIP code, party affiliation, the last four digits of their Social Security number, and their driver’s license number.

He said investigators have told the board the hack was an outright ”attack - like they were looking for something.”

Mr. Miller said the invasion of the county’s computer system has been reported to the FBI, the Ohio Secretary of State, the Ohio Attorney General, and U.S. Homeland Security. It is still under investigation by the FBI.

He said the county had had its system analyzed ”a couple of times” earlier this year, but the hackers apparently exploited a previously unknown vulnerability. He said the computer attack was ”much more complex” than having a password.

Joshua Eck, a spokesman for the Ohio Secretary of State’s office, said the office shut down its connection with Henry County to replace the existing computer that was used to link the two agencies and helped the board rebuild its database.

Contact Tom Troy: tomtroy@theblade.com or 419-724-6058 or on Twitter @TomFTroy.

First Published December 6, 2016, 9:35 p.m.