The last four Social Security numbers of 463,000 registered voters in Pierce County were accidentally given last December to a person who asked for voter registration records from county election officials.

The release of what usually would be information kept secure from the general public was revealed in letters that were mailed in mid-January to the affected voters by the county auditor, who oversees local elections.

The data that included Social Security numbers was provided by the auditor’s election department in response to a public records request, a formal process in which citizens and the news media can obtain certain records maintained by governments that aren’t easily made available to the public.

The names, addresses and birthdates of voters that were released is public information but shouldn’t have included the Social Security digits. The person who made the request, and whose own identity wasn’t revealed by officials, didn’t want the sensitive personal information, didn’t save it, and deleted it after receiving it, according to the auditor’s office.

The partial Social Security numbers, which are recorded by the auditor as a way to confirm the identity of registered voters, were accidentally included in a list of voters released by an employee of the auditor. Under federal and state laws, Social Security numbers, including partial ones, can’t be part of public-record requests.

“This kind of mistake has never happened before” in the auditor’s office, and steps have been taken to prevent a recurrence, Whitney Stevens, a public records officer, wrote in the auditor’s letter to voters.