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.