The federal government’s decision to pay out billions of dollars in tax credits to illegal immigrants likely was made by midlevel bureaucrats and has never received full congressional scrutiny, according to a study that the Center for Immigration Studies is releasing Monday.

The report, written by CIS fellow David North, says the Internal Revenue Service doled out $4.2 billion in what is known as the “additional child tax credit” in 2010 to those using an individual taxpayer identification number, or ITIN, which is usually a signal of an illegal immigrant.

The issue has been known for some time. But Mr. North went deep into the data to try to look at why it’s happening, and said it’s a story of a tax credit expanding beyond its initial scope, and midlevel IRS managers twisting the law, leaving billions of dollars going to illegal immigrants.

In particular, he said he can find no evidence that the decision to allow illegal immigrants to collect the tax was made by top leaders at the IRS, and he said it wasn’t the intent of Congress.

“I’ve been in government and I know kind of how these things work out. It struck me that [GS-]15s, 16s, got together at some point and decided this is how we should handle it, and it stuck,” Mr. North said.

Nobody was answering the phones at the IRS during the government shutdown, which began Oct. 1.