A senator wants Congress' investigative arm to determine whether the Transportation Department has broken the law by spending federal money on a program allowing Mexican trucks on U.S. roads.

Sen. Byron Dorgan, D-N.D., called for the investigation by the Government Accountability Office a few hours after Transportation Secretary Mary Peters warned of economic losses if Mexican trucks are prohibited from driving deep into the U.S.

Peters has been fighting in court to prevent the program's end. But Dorgan and others say Congress prohibited spending money on the program last year.

"When Congress passes a law that says no funds can be used for this program, we mean no funds can be used for this program," Dorgan said in a news release. "The Department of Transportation cannot simply pick and choose which laws they want to follow and which laws they want to break."

Dorgan said the agency is violating the Antideficiency Act, which prohibits spending federal money that has not been authorized or appropriated.

The North American Free Trade Agreement gave Mexican trucks greater access to U.S. roads beginning in 1995. But the U.S. only opened the roads to a few trucks when the pilot program began last September.

Long-standing opposition from labor and safety groups had kept the trucks off most U.S. roads. Without the program, Mexican trucks are confined to about 25 miles beyond the border, where goods they bring are picked up by U.S. truck drivers.

Peters said Monday the agency is not violating the law. The law prohibits using funds to establish the program, she said, but the money is being used on the existing program. The agency has made similar arguments in the 9th U.S. Circuit Court of Appeals, which is considering an appeal by the International Brotherhood of Teamsters to stop the program.