US authorities said Monday a breach of taxpayer records was more extensive than originally estimated, and that some 300,000 accounts appear now to have been affected.
The Internal Revenue Service said it identified some 220,000 taxpayers whose tax records could have been stolen through its online "get transcript" system, which was shut down after news of the breach.
The agency said in May the breach affected some 100,000 taxpayer accounts but on further review said it had "identified more questionable attempts to obtain transcripts using sensitive information already in the hands of criminals."
"As it did in May, the IRS is moving aggressively to protect taxpayers whose account information may have been accessed," said a statement on the agency's website.
"The IRS will begin mailing letters in the next few days to about 220,000 taxpayers where there were instances of possible or potential access to 'Get Transcript' taxpayer account information."
The agency said it identified another 170,000 taxpayer accounts where unauthorized parties tried but failed to fain access, in addition to some 111,000 accounts identified earlier this year.
The news comes after US officials confirmed some 21 million US government employees, contractors and others had personal information compromised in two breaches.
Officials have declined to identify the source of the hack of personnel records but it is widely believed to be part of a Chinese espionage program.