Thieves used an online service provided by the Internal Revenue Service to gain access to information from more than 100,000 taxpayers, the agency said Tuesday.
Officials said the information included tax returns and other tax information on file with the IRS.
The IRS said the thieves accessed a system called "Get Transcript.'' In order to access the information, the thieves cleared a security screen that required knowledge about the taxpayer, including Social Security number, date of birth, tax filing status and street address.
"We're confident that these are not amateurs,'' said IRS Commissioner John Koskinen.
Koskinen said the agency was alerted to the thieves when technicians noticed an increase in the number of taxpayers seeking transcripts. The IRS said they targeted the system from February to mid-May. The service has been temporarily shut down.
Taxpayers sometimes need copies of old tax returns to apply for mortgages or college aid. While the system is shut down, taxpayers can still apply for transcripts by mail.
The IRS said its main computer system, which handles tax filing submissions, remains secure.
The IRS has launched a criminal investigation. The agency's inspector general is also investigating.
"In all, about 200,000 attempts were made from questionable email domains, with more than 100,000 of those attempts successfully clearing authentication hurdles,'' the agency said. "During this filing season, taxpayers successfully and safely downloaded a total of approximately 23 million transcripts.''
The agency is still determining how many fraudulent refunds were claimed this year using information from the stolen transcripts. Koskinen provided a preliminary estimate, saying less than $50 million had been successfully claimed.
However, thieves can use the information to claim fraudulent tax refunds in the future. As identity theft has exploded, the agency has added filters to its computer system to identify suspicious returns. These filters look for anomalies in the information provided by the taxpayer.
Old tax returns can help thieves fill out credible looking returns in the future, helping them get around the IRS filters.
This year, the IRS stopped almost 3 million suspicious returns, Koskinen said.
Tax returns can include a host of personal information that can help someone steal an identity, including Social Security numbers and birthdates of dependents and spouses. The IRS said the thieves appeared to already have a lot of personal information about the victims.
The IRS said it was notifying taxpayers whose information had been accessed.