From their Huntington Beach, Calif., facility, Boeing Co. engineers crafted a plan to line the Mexican border with 1,800 towers equipped with sensors that can spot when illegal immigrants step onto U.S. soil.

Infrared cameras will detect the body heat of intruders, and radar will track vehicles used to smuggle immigrants and drugs into the United States.

On Thursday, Boeing's plan won a key contract, one that could lead to a "virtual fence" along 7,500 miles of the U.S.'s borders with Mexico and Canada.

Department of Homeland Security officials said Thursday that the initial three-year contract was worth $67 million and called for Boeing to build its tower-based system along a 28-mile stretch of the border south of Tucson. But analysts said the project could be expanded and total more than $2.5 billion.

Boeing beat out three of the nation's largest defense corporations and a Swedish cellphone maker for the contract - part of a larger homeland security initiative to stem the flow of undocumented immigrants into the country.

When the project is completed, Boeing said, its towers would leave little of the now-porous borders undetected, creating a high-tech fence longer than the Great Wall of China.

"What we are looking to build is a 21st-century virtual fence," said Homeland Security Secretary Michael Chertoff at a news conference announcing the contract.

The contract isn't without controversy. Some members of Congress have called it a waste of money, noting that two other unsuccessful efforts to control the Mexico border with high-tech equipment have cost taxpayers more than $500 million.

Sen. Patrick J. Leahy, D-Vt., criticized plans to build a barrier along the northern border with Canada.

"I have heard some cockamamie ideas in my time in the Senate, but this one rises to the top," he said in a statement.

Federal officials declined to say how much the full-scale system would ultimately cost. But analysts said that eventually the security initiative could be worth more than $8 billion as the U.S. looks to expand the project to secure the East and West coastlines.