The Internal Revenue Service (IRS) said on Friday an investigation into a hacking attack revealed in May found that cyber criminals had made attempts to gain access to about 390,000 additional taxpayer accounts.
The tax agency said 295,000 taxpayer transcripts were also targeted but access was not successful.
The agency said in May the tax return information of about 114,000 U.S. taxpayers had been illegally accessed by cyber criminals over the preceding four months, with another 111,000 unsuccessful attempts made.
The IRS then revealed in August a new review had identified 220,000 additional incidents where data was breached and another 170,000 suspected failed attempts to gain access to taxpayer data.
Mailings to the affected taxpayers will start on Feb. 29, IRS said.