The IRS acknowledged Friday that cyber hackers have stolen Social Security numbers and other information from more than 700,000 taxpayers -- roughly double the number the agency previous estimated.

The cyber thieves hacked into the agency’s “Get Transcripts” system in which taxpayers get returns and other previous-year filings.

The breach, believed to have been carried out in Russia by a criminal operation, was discovered in May 2015, and the increase was reported first by The Wall Street Journal.

The IRS originally said information was taken from about 113,000 taxpayers.

The agency said it has notified the additional roughly 390,000 taxpayers and is giving them special passwords and offering them services for identity-theft protections.

"The IRS is committed to protecting taxpayers on multiple fronts against tax-related identity theft," said agency Commissioner John Koskinen. "We are moving quickly to help these taxpayers."

The IRS said the additional discoveries were made when the Treasury Inspector General for Tax Administration conducted a nine-month-long investigation that looked as far back as the launch of the “Get Transcripts” web application in January 2014.

Hackers tried unsuccessfully to access information from an additional 295,000 additional IRS accounts, the agency also said.

The Associated Press contributed to this report.