WASHINGTON (AP) — The Homeland Security Department swept aside evaluations of government experts and named Mississippi — home to powerful U.S. 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 it scored high than Mississippi.

‘‘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 U.S. livestock industry. They assigned the site a score that ranked it 14th among 17 candidate sites in the United States.

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. Cohen accepted the argument that, ‘‘When built, they come.’’

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

For Wisconsin, 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,’’ 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. 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.

Thompson said he never spoke about the subject with Cohen. But the department said Thompson met with Cohen at least twice and discussed plans for the new lab, once in February 2007 in Mississippi and again a year later in Washington.

‘‘You told me more about the process than I know,’’ Thompson told the AP. ‘‘I haven’t talked to anyone about it, not to Jay Cohen or anyone.’’

Flora, Miss., is not in Thompson’s congressional district. But the consortium of public and private organizations working to attract the lab includes Tougaloo College, where Thompson received his bachelor’s degree, and Jackson State University, where he was awarded his master’s degree.

A spokeswoman for Cochran, Margaret McPhillips, denied that the department relied on the scoring system described in the documents obtained by the AP. She dismissed it as rumor.

‘‘Our congressional delegation doesn’t know about a scoring system,’’ McPhillips wrote in an e-mail to the AP. ‘‘Mississippi’s governor does not know of one. DHS is in Mississippi right now for a site visit and just confirmed with us that there is no scoring system.

‘‘Mississippi has put forth a compelling application and it does not surprise me that someone might be trying to diminish the strength of our proposal by spreading this rumor,’’ McPhillips wrote.

Some lawmakers already skeptical over the department’s plans said Cohen’s intervention on behalf of Mississippi appears improper.

‘‘It appears that the undersecretary responsible for this program may have corrupted the site selection process by putting his thumb on the scale in favor of a particular site and its contractor, in violation of his own rules and over the objections of his own advisers,’’ said Rep. John Dingell, D-Mich. ‘‘This raises the question of whether DHS is interested in bioresearch or just shameless empire building.’’

Dingell, chairman of the House Energy and Commerce Committee, held oversight hearings in May examining the risks of building the new lab on the U.S. mainland near livestock herds. The facility would replace an existing 24-acre research complex on isolated Plum Island, about 100 miles northeast of New York City in the Long Island Sound. Besides foot-and-mouth disease, researchers also would study African swine fever, Japanese encephalitis, Rift Valley fever and the Hendra and Nipah viruses. Construction would begin in 2010 and take four years.

‘‘If any of the five finalists scored lower than those eliminated from the process, we’ve got a big problem on our hands,’’ said Sen. Claire McCaskill, D-Mo. Her state’s proposal also scored higher than Mississippi’s, but Homeland Security rejected it over ‘‘growing negative community feedback.’’

In his memo, Cohen acknowledged the government evaluation committees graded Mississippi’s site as merely ‘‘satisfactory’’ with scores of 72 and 75 in its research and work force categories, respectively. The Mississippi site’s overall grade was 81, or ‘‘very good,’’ which still was lower than nine other rejected U.S. sites.

‘‘While I take the committees’ concerns to heart, I do not concur with the low scores,’’ Cohen wrote.

A department spokeswoman, Amy Kudwa, said the agency’s internal committee reviews ‘‘did not appropriately consider the unique contributions certain consortia committed to make in their proposals.’’ Mississippi, for example, promised to work closely with Battelle Memorial Institute, a Homeland Security contractor that already manages some national labs elsewhere for the Homeland Security and Energy departments.

Besides Flora, Miss., the U.S. locations under consideration for the new lab are Athens, Ga.; Manhattan, Kan.; Butner, N.C.; and San Antonio.

The nine sites rejected as finalists that also earned high scores than Mississippi’s location were: Leavenworth, Kan.; a different location in Athens, Ga.; two other sites in San Antonio; the University of Missouri in Columbia, Mo.; Beltsville; College Station, Texas; Madison; and Tracy, Calif., near the Lawrence Livermore National Laboratory.