Nearly 2,500 refugees from terrorism hotspots around the world are bound for the U.S. after being rejected by Australia, but not even top lawmakers can get answers about who they are.

In an unprecedented move, the U.S. State Department has classified details on refugees to be resettled in America via a secret deal made with Australia. The bi-lateral agreement, which Australian Prime Minister Malcolm Turnbull called a “one-off,” involves 2,465 people currently being held in Papua New Guinea and Nauru who will now be transferred onto U.S. soil.

“This is a backroom deal, wheeling and dealing with another country's refugee problem,” Center for Immigration Studies fellow Don Barnett told FoxNews.com. “I don’t believe for a moment it’s a one-time deal. That’s for public consumption.”

The move has also raised a red flag among Congressional oversight members.

In a letter to Department of Homeland Security Secretary Jeh Johnson and Secretary of State John Kerry, key lawmakers Sen. Chuck Grassley, R-Iowa, and Rep. Bob Goodlatte, R-Va., complained about the lack of transparency.

“This situation is concerning for many reasons,” read the letter, charging that “your departments negotiated an international agreement regarding refugees without consulting or notifying Congress.”

Screeners from the U.S. Customs and Immigration Services are set to leave for the Pacific Island nations next month to begin vetting the refugees.

When staffers probed the number of individuals being considered for resettlement, they were told it was “classified,” even though refugee admissions are traditionally public. Officials, however, did confirm countries of origin to be Iran, Sri Lanka, Pakistan, Afghanistan, Somalia, Iraq and Sudan, as well as some deemed “stateless.”

Iran, Sudan and Syria are the three countries on the U.S. current State Sponsors of Terrorism list. But Barnett said the “stateless” category is most worrisome.

“These could be Burmese Muslims, who have posed assimilation issues for every nation which has taken them,” said Barnett. “It’s a dangerous precedent which says, ‘We’ll take any ethnic group with which you don’t get along.’”

Australia has been under fire for paying poor surrounding island nations to house detention centers for refugees. Australia created the camps in an effort to curtail “people smuggling” and has long had a policy which prevents individuals seeking asylum from entering the country before being vetted.

In a statement, the State Department said the Obama administration is proud of its role in taking in refugees, even ones other nations don't want.

“The United States is proud of its long history as the largest refugee resettlement country in the world," read the statement. "As the President has announced, our refugee resettlement program has grown substantially in the last year.

"The United States has agreed to consider referrals from the UN High Commissioner for Refugees (UNHCR) of refugees now residing in Nauru and Papua New Guinea," the statement continued. "These refugees are of special interest to UNHCR and we are engaged on a humanitarian basis, as we are in other parts of the world."

The Goodlatte-Grassley letter also asked why Australia and other countries refused to take in the refugees.

“If they’ve been vetted and deemed inadmissible, the U.S. can’t say, ‘You don’t want them, so we’ll take them,’” said Barnett.

Speaking at a Nov. 14 press conference, Turnbull said, “Nobody is taking any more refugees, but what the Americans are doing is assisting these individuals on Nauru and Manus by bringing them in within their existing quota.”

The Obama administration increased the quota for the 12-month period that began in October to 110,000 refugees, up from 85,000 the previous year.

Turnbull’s announcement that his country would be “taking more refugees from Central America” as part of some “commitments at President Obama's Refugee Summit” has also sparked speculation that the deal is a trade of refugees from the most dangerous areas of the world for ones from Central America.

In July, Costa Rica agreed to set up a detention center for those wishing to enter the United States through a new program the administration established initially for children to reunite with U.S. -based parents. The program has been expanded to include those fleeing gang violence in Guatemala, Honduras and El Salvador. It also created a special category for female heads of households.

“All you have to do is say is you’re a single mother and you’re in,” said Barnett. “There is all sorts of twisting of the law here.”

A “particular social group,” is one of the five categories applicants must meet to obtain refugee or asylum status. The designated group could be a religion, nationality, race or political group whose members have a “well-founded fear” of persecution for belonging.

While U.S. law stipulates refugee status should not be granted based on generalized violence, according to officer training materials obtained by FoxNews.com, status can be considered for women heads of households who are vulnerable to crime, economic hardships or could be a victim of “Machista,” described in materials as a “cultural pattern where men father children, [then] abandon [their] family.”

The Office of the United Nations High Commissioner for Refugees estimates that 146,000 individuals will apply to enter the U.S. through the Costa Rican Center.

In a July 26, 2016 press release the State Department laid out the logistics in which after pre-screening and transfer to Costa Rica, “they will undergo refugee processing before being resettled to the United States or another third country.”

Representatives from the Grassley-Goodlatte committee are set to receive a classified briefing on the Australian refugee deal next week. However, numerous questions could remain shielded from public view including costs, timing, benefit to the U.S. and perhaps most importantly why it was done in secret.