John McKinnon and Laura Saunders write that the new estimate covers an extended period of review from November last year to May, when the agency first reported the breaches.
The report said the total number of people targeted, including those whose accounts hackers tried but failed to access, is approximately 615,000.
The data breaches focused on the Get Transcript online application, which provides taxpayers access to prior-year tax data that identity thieves can use to file for fraudulent returns, McKinnon and Saunders write.
“The IRS takes the security of taxpayer data extremely seriously, and we are working to continue to strengthen security for Get Transcript, including by enhancing taxpayer-identity authentication protocols,” an agency statement said, according to the report.
IRS noted in May that it plans to provide free credit protection and other services to the affected people to help prevent tax refund fraud, the report said.