TIGTA also found hackers attempted to access the tax return transcripts of another 295,000 people during a further breach investigation that lasted for nine months, IRS said Friday.
The agency halted operations of its Get Transcript application since it discovered the hack in May last year and will send notification mails to the 685,000 individuals beginning Monday.
“IRS is committed to protecting taxpayers on multiple fronts against tax-related identity theft, and these mailings are part of that effort,” noted John Koskinen, commissioner of the IRS.
Individuals whose data were compromised in the agency hack are eligible to receive a one-year identity theft protection service from Equifax.
The agency initially found 334,000 tax return transcripts were accessed by hackers and 281,000 were targeted but not accessed.