Gov. Chris Christie approved a law firm with expertise in federal transit issues to help challenge a $271 million tab the federal government says the state owes for a canceled rail tunnel.

Christie's office said the governor had signed off Thursday on the selection of the Washington, D.C., firm of Patton Boggs. Rodney Slater, who served as U.S. transportation secretary under President Bill Clinton, and former Republican U.S. Sen. Trent Lott are among the partners.

"We're delighted to have been engaged by the state," said Stuart Pape, the firm's managing partner. The firm, which has an office in Newark, is assessing its strategy, Pape said.

Christie killed the $8.7 billion tunnel from New Jersey to New York City on Oct. 27, citing potential cost overruns that he said could add $2 billion to $5 billion or more to the price.

The federal government and Port Authority of New York and New Jersey each contributed $3 billion to the project, while New Jersey's share was $2.7 billion. The state and Port Authority were responsible for overruns.

Christie is fighting a bill for the return of federal money already spent on engineering and construction of the tunnel. The Nov. 24 bill from the Federal Transit Authority seeks payment within 30 days.

"It's not surprising that the same federal transit agency that had no clear way to pay for cost overruns of a project already hurt by poor planning and inequitable cost sharing is relying on bureaucratic power plays to wring even more money out of New Jerseyans," Christie said in a statement Thursday.