Officials insist the errors were limited to 23,000 of the 1.4 million voter registration files sent to elections offices between late April, when California’s new automated “motor voter” system went into effect, and early August. Californians who were affected will soon receive notifications in the mail instructing them to check their voter registration status.

Jean Shiomoto, the state’s DMV director, and Amy Tong, director of the California Department of Technology, described the problem as “an administrative processing error” in a letter to Secretary of State Alex Padilla, California’s chief elections officer.

“We are committed to getting this right and are working closely with the Secretary of State’s office to correct the errors that occurred,” Shiomoto said in a written statement. Tong declined to comment beyond the letter.

The errors, which were discovered more than a month ago, happened when DMV employees did not clear their computer screens between customer appointments. That caused some voter information from the previous appointment, such as language preference or a request to vote by mail, to be “inadvertently merged” into the file of the next customer, Shiomoto and Tong wrote. The incorrect registration form was then sent to state elections officials, who used it to update California’s voter registration database.

There’s shorter waits at California’s DMVs, but agency says there’s still work to do »

A small number of the mistakes — officials estimated around 1,600 — involved people who did not intend to register to vote. State officials said no people in the country illegally — who are eligible to get a special driver’s license in California — were mistakenly registered to vote. An unknown number of errors included voters whose political party preferences were changed without their consent. Officials did not provide additional details about the errors they uncovered during a monthlong investigation.

State officials said they found the errors by comparing DMV records, which had the correct information, with what was sent to elections officials. Californians also undoubtedly saw mistakes in their mailbox: Anyone who registers to vote is sent a card with their registration information.

Officials said Wednesday that a fix has been put in place to prevent additional mistakes.

The mistakes came less than four months after other problems surfaced with the rollout of California’s motor voter system, mandated by state law to register any U.S. citizen to vote who applies for or renews a license at the DMV. That error — potentially thousands of cases in which multiple registration documents were generated for a single voter — was caught before the June primary. The timing of the new error, officials said, should not cause problems for any Californian who changes registration information before the November election. County elections officers across the state were also briefed Wednesday.

Padilla, who said he was “extremely disappointed and deeply frustrated” with the mistakes, said he remains confident in the ambitious new voter registration program.

“I hope this doesn’t detract from the otherwise overwhelming success that motor voter has been,” he said.

The mistake also comes at an inauspicious time for California’s motor vehicles agency, which is the focus of widespread criticism for long lines that formed at many of its field offices over the summer. Officials have scrambled to bring in several hundred additional workers to shorten the waits, the agency announced Wednesday ahead of the announcement about the voter registration mistakes.

In the letter sent to affected DMV customers, Shiomoto said she “sincerely apologizes for this inconvenience” and urged voters to check their registration status online.

john.myers@latimes.com

Twitter: @johnmyers

UPDATES:

7:25 p.m.: This article was updated with additional information that clarifies how many new registrations were created by the DMV error.

This article was originally published at 4:50 p.m.