Nearly a million illegal immigrants got work permits in the first six years of the Obama administration, the Center for Immigration Studies says in a report. Among the illegal immigrants who obtained work permits were people who were denied asylum, were suspected of using fraudulent documents or were stowaways, the conservative research group said.

“I was astonished at the huge number of work permits that are being issued by the Obama administration outside the legal immigration system through executive discretion, especially at a time of high unemployment and stagnant wages,” Jessica Vaughn, the center’s director of policy studies, said in a statement. “Besides the effect on the American worker, it encourages and rewards more illegal immigration.”

U.S. Sen. Jeff Sessions, R-Ala., said the findings by CIS, which obtained the data from U.S. Citizenship and Immigration Services, “unearthed the operation of a shadow immigration system previously unknown to the American public” and warrants a “full investigation.”

“This massive increase in the labor supply has occurred simultaneously with a steep drop in family incomes and a sharp rise in the number of Americans pushed out of the workforce. All jobs gains since the recession have gone to foreign workers, while the slack labor market has depressed median family incomes almost $5,000 in that time,” he said in a statement.

Of the 5.5 million work permits awarded to noncitizens from 2009 to 2014, about 982,000 were issued to illegal aliens or “aliens unqualified for admission” to the U.S., according to CIS. Most of those, or 957,000 of them, were doled out to people who illegally crossed the border, the group said.

Another 1.7 million work permits were given to people “whose status was unknown, not recorded by the adjudicator, or not disclosed by USCIS,” the report found. “This should be a concern; work permits are gateway documents to driver’s licenses and other benefits, and if the government agency issuing them does not know or will not disclose how the bearer arrived in the country, how can others rely on the authenticity of an individual’s identity? It is equally disconcerting if the government does know and chooses not to disclose it.”

About two-thirds of the 1.8 million new work permits given to foreigners with temporary visas were issued to those who aren’t eligible to work in the U.S., according to the group. About 470,000 permits were given to people on tourist visas and 532,000 were doled out to foreign students, according to CIS data. The children or dependents of guest workers were also given some 156,000 work permits even though they also are not eligible to work in the country, the group said.