From an ABC report at the link below:
Thieves used an online service provided by the IRS to gain access to information from more than 100,000 taxpayers, the agency said Tuesday.
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.
Full report here.