Cyberattacks on taxpayer accounts may have affected more people than previously reported, the Internal Revenue Service said Friday.
The IRS statement, originally reported by Dow Jones, revealed tax data for about 700,000 households might have been stolen: Specifically, a government review found potential access to about 390,000 more accounts than previously disclosed.
In August, the IRS said that the number of potential victims stood at more than 334,000 — more than twice the initial estimate of more than 100,000.
"If somebody has all this information … we may see [a] resurgence next year of fraudulent tax returns," Paul Stephens, director of policy and advocacy for the Privacy Rights Clearinghouse, told CNBC in 2015.
The IRS discovered an incident involving its "Get Transcript" application last May, and the Treasury Inspector General for Tax Administration conducted a nine-month investigation. That review turned up the additional accounts that could potentially have been accessed.
Additionally, the IRS said there were 295,000 taxpayer transcripts that were targeted, but "access was not successful."
The agency said it will send mailings to affected taxpayers beginning February 29.
"The IRS is committed to protecting taxpayers on multiple fronts against tax-related identity theft, and these mailings are part of that effort," IRS Commissioner John Koskinen said in a statement. "We appreciate the work of the Treasury Inspector General for Tax Administration to identify these additional taxpayers whose accounts may have been accessed. We are moving quickly to help these taxpayers."
— CNBC's Kelli Grant contributed to this report.