WASHINGTON -- Sophisticated criminals used an online service run by the IRS to access personal tax information from more than 100,000 taxpayers, part of a scheme to steal identities and claim fraudulent tax refunds, the IRS said Tuesday.

The thieves accessed a system called "Get Transcript," where taxpayers can get tax returns and other filings from previous years.

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, the IRS said.

"We're confident that these are not amateurs," said IRS commissioner John Koskinen. "These actually are organized-crime syndicates that not only we but everybody in the financial industry are dealing with."

Koskinen wouldn't say whether investigators believe the criminals are based overseas or where they obtained enough personal information about the taxpayers to access their returns.

The IRS has launched a criminal investigation.

The agency's inspector general also is investigating.

Identity thieves have stepped up efforts in recent years to claim fraudulent tax refunds.

The agency estimates it paid $5.8 billion in fraudulent refunds to identity thieves in 2013.

Congress is pressing the IRS for information about the breach.

"That the IRS -- home to highly sensitive information on every single American and every single company doing business here at home -- was vulnerable to this attack is simply unacceptable," said Sen. Orrin Hatch, R-Utah, chairman of the Senate Finance Committee.