FILE: June 18, 2014: U.S. Customs and Border Protection at the Customs and Border Protection Nogales Placement Center in Nogales, Ariz. (AP)

The Obama administration has since 2009 issued roughly 5.5 million work permits to non-citizens beyond what Congress has authorized, according to recently-released documents that critics of U.S. policy say reveals a "shadow" or "parallel" immigration system stifling wages and taking jobs from Americans.

The information was obtained by the conservative-leaning Center for Immigration Studies through a Freedom of Information Act request and has prompted Alabama GOP Sen. Jeff Sessions to call for an investigation.

"This request has unearthed the operation of a shadow immigration system previously unknown to the American public," said Sessions, one of Capitol Hill's most outspoken critics of President Obama's immigration policy. "A full investigation is warranted."

Congress authorized an estimated 5 million green cards and 3.5 million guest worker permits during the 2009-2014 period, in addition to the 5.5 million issued by administration action, a Senate staffer said Wednesday.

Jessica Vaughn, the study author and the center's director of policy studies, argues the administration has discovered the power to issue work permits outside the limits set by Congress and that it has become "the vehicle" for Obama’s executive actions -- in which he has offered deferred deportation to millions of people now in the country illegally.

The biggest group of recipients has been people entering the U.S. without being inspected. More than 957,200 of them received permanent or "pre-permanent" work permits, according to the center.

Others received those types of permits were 23,215 parolees, nearly 1,000 stowaways and 49 people suspected of document fraud.

In addition, 531,692 students and 470,028 students received temporary work permits over that period. And those in both groups were neither originally admitted to the U.S. for employment nor qualified for admission, the group found.

"Some of those people are on track to get a green card," Vaughn said recently on Fox News Business' "Lou Dobbs Tonight." "But the vast majority of them entered illegal or on a tourist visa or the visa waiver program. … It's not like there’s a labor shortage here."

Session has largely framed his argument against Obama's immigration policy as hurtful to U.S. workers, many of whom have not seen wages increase in the growing, post-recession economy.

"The slack labor market has depressed median family income by $5,000" since 2009, he said.

Vaughn also broke down the numbers to show the three biggest groups ineligible for work permits but receiving them are illegal immigrants (928,000), people of "unknown" immigration status (1.7 million), and those on a temporary visa (1.8 million).

"There's no reason to issue (work permits) to people here illegally or whose status is unknown," Vaughn also said.

She said 1.7 million have either not been recorded or their statuses have not being disclosed by the Citizenship and Immigration Services, which  should be a concern because work permits are “gateway documents” to drivers' licenses and other benefits.

The agency did return a call requesting comment.

"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," Vaughn said.