The IRS is struggling to ensure that illegal immigrants are able to illegally use Social Security numbers for legitimate purposes, the agency's head told senators on Tuesday, without allowing the numbers to be used for "bad" reasons.

IRS Commissioner John Koskinen made the statement in response to a question from Sen. Dan Coats, R-Ind., during a session of the Senate Finance Committee about why the IRS appears to be collaborating with taxpayers who file tax returns using fraudulent information. Coats said that his staff had discovered the practice after looking into agency procedures.

"What we learned is that ... the IRS continues to process tax returns with false W-2 information and issue refunds as if they were routine tax returns, and say that's not really our job," Coats said. "We also learned the IRS ignores notifications from the Social Security Administration that a name does not match a Social Security number, and you use your own system to determine whether a number is valid."

Asked to explain those practices, Koskinen replied, "What happens in these situations is someone is using a Social Security number to get a job, but they're filing their tax return with their [taxpayer identification number]." What that means, he said, is that "they are undocumented aliens … . They're paying taxes. It's in everybody's interest to have them pay the taxes they owe."

As long as the information is being used only to fraudulently obtain jobs, Koskinen said, rather than to claim false tax returns, the agency has an interest in helping them. "The question is whether the Social Security number they're using to get the job has been stolen. It's not the normal identity theft situation," he said.