Hackers may have accessed the tax transcripts of approximately 724,000 United States taxpayers by using stolen personal information, the Internal Revenue Service announced Friday.
The agency also said hackers targeted another 576,000 accounts, but could not access them.
The announcement followed a nine-month investigation into its “Get Transcript” application.
The tool was launched in January 2014 and gave taxpayers a way to download or order several years of their transcripts through the IRS website.
However, the agency announced last May that “criminals” had been able to access other tax histories that were not their own by using personal information that had been stolen elsewhere.
The IRS originally announced that about 114,000 transcripts may have been improperly accessed, while hackers targeted another 111,000 but were unsuccessful in their attempts.
The tool has been offline ever since while officials searched for other suspicious activity.
The Treasury Inspector General for Tax Administration (TIGTA) has handled the investigations.
In August, the IRS announced TIGTA found about another 220,000 cases of potential breaches since “Get Transcript” debuted, and about 170,000 more unsuccessful suspicious attempts.
On Friday, the IRS announced TIGTA’s latest review found about 390,000 potential additional cases of improper access, and some 295,000 cases where tax data was targeted but not obtained.
The IRS noted that some of the attempts might not have been malicious.
“It is possible that some of those identified may be family members, tax return preparers or financial institutions using a single email address to attempt to access more than one account,” it said in a statement, though added it is notifying all of the affected taxpayers as a precaution.
The latest wave of taxpayers will be notified through the mail beginning Feb. 29, the IRS said.
“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 Friday’s announcement. “We appreciate the work of the Treasury Inspector General for Tax Administration to identify these additional taxpayers whose accounts may have been accessed.”
The agency is offering all affected taxpayers free identity theft protection services and the chance to obtain an identity protection PIN, which helps protect Social Security numbers on returns.