n"">Tax return information for about 100,000 U.S. taxpayers was illegally accessed by cyber criminals over the past four months, U.S. IRS Commissioner John Koskinen said on Tuesday, the latest in a series of data thefts that have alarmed American consumers.

From February to May, attackers sought to gain access to personal tax information 200,000 times through the agency's "Get Transcript" online application, which calls up information from previous returns, he told a news conference. About half of those attempts were successful.

The breach did not affect any IRS data outside the "Get Transcript" application, and the agency said it would strengthen its security measures.

Koskinen said he could not comment on who the attackers might be, and a criminal investigation was ongoing.

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

The data theft was largely intended to steal taxpayers' information to submit fraudulent returns next year, he said.

The agency currently believes that fewer than 15,000 fraudulent returns were processed as a result of the breach, likely resulting in refunds of less than $50 million.