Thieves hacked into an Internal Revenue Service online service and gained access to information from more than 100,000 taxpayer accounts, the agency said Tuesday.
The criminals used personal data obtained from other sources, including Social Security numbers, street addresses and dates of birth, to get into the IRS "Get Transcript" service. The service has been temporarily shut down.
The thieves gained access to tax returns and other tax information on file with the IRS. The IRS' main computer system that handles tax filings wasn't compromised.
The IRS said in a statement that the hack was extremely complex, requiring specific, personal knowledge of the taxpayers, including answering personal identification questions. It occurred from February through mid-May. The IRS first detected unusual activity last week.
"As always, the IRS takes the security of taxpayer data extremely seriously, and we are working aggressively to protect affected taxpayers and continue to strengthen our protocols," the IRS said.