WASHINGTON — The Homeland Security Department swept aside evaluations of government experts and named Mississippi — home to powerful American lawmakers with sway over the agency — as a top location for a new $451 million, national laboratory to study some of the world’s most virulent biological threats, according to internal documents obtained by the Associated Press.

Mississippi’s lawmakers include the Democratic chairman of the department’s oversight committee in the House and the senior Republican on the Senate Appropriations Committee, which is expected to approve money to build the National Bio and Agro-Defense Facility at one of five sites being considered. The two lawmakers said they were unaware of the Homeland Security evaluation system that scored the Mississippi site so low.

The disclosure is the latest example of what critics assert is the Bush administration’s politicizing of government decisions, such as efforts to steer science over global warming at the Environmental Protection Agency and hiring and firing practices at the Justice Department.

“It is very suspicious,” said Irwin Goldman of the University of Wisconsin, a leader of the unsuccessful effort to build the lab in Madison. His community’s offer was among nine sites rejected even though the government scored it more highly than Mississippi’s. “We wondered how everybody else did. It’s interesting to know that we came out ahead of one that was short-listed.”

The states where locations were eliminated despite earning scores higher than Mississippi include California, Georgia, Maryland, Missouri, Texas, and Wisconsin.

Government experts originally expressed concerns that the proposed site in Flora, Miss., was far from existing biodefense research programs and lacked ready access to workers already familiar with highly contagious animal and human diseases, such as foot-and-mouth virus, that could devastate the American livestock industry. They assigned the site a score that ranked it 14th among 17 candidate sites in America.

But a senior Homeland Security official, Undersecretary Jay Cohen, overruled those concerns under the theory that skilled researchers would move to Mississippi if it were selected for the new lab, according to a July 2007 internal government memorandum, marked “sensitive information” and obtained by the AP. Mr. Cohen accepted the argument that, “When built, they come.”

A former Navy officer, Mr. Cohen is a political appointee, nominated by President Bush in June 2006.

For Wisconsin, Mr. Cohen determined that community opposition to the new lab was too great despite the area’s highly respected researchers. Some local officials had threatened to withhold sewer service from the lab.

“It raised my eyebrows a bit when Mississippi was selected,” said George Stewart of the University of Missouri, another rejected location that also earned a score higher than Mississippi’s. “Obviously, there were factors other than what they were looking in the site visits. The group that did the site visits were scientists and know what they were looking for. I don’t know what DHS was looking for.”

Stephen Schimpff, who led unsuccessful efforts to bring the lab to Beltsville, Md., complained that the government’s analysis seemed confusing. The department said there were too many skilled researchers near Beltsville, just outside Washington, and the agency worried about competing to hire them.

“We were surprised when some of the things we felt were our strengths were turned back on us as weaknesses,” Mr. Schimpff said.

Under the department’s own rules, it was free to disregard the recommendations of the government experts it appointed. But it said it selected advisers who were experts and were screened carefully for any conflicts of interest, working through seven stages of recommendations over 18 months. Mr. Cohen personally made the choices for the five sites in the eighth and final stage of the decision.

Mississippi’s lawmakers include Democratic Rep. Bennie Thompson, chairman of the House Homeland Security Committee, and Sen. Thad Cochran, the top Republican on the Senate Appropriations Committee and the subcommittee that oversees Homeland Security money. Each said he was not aware of the department’s deliberations.