WASHINGTON-Homeland Security Secretary Janet Napolitano told lawmakers Wednesday that the agency cannot meet its 2012 deadline for radiological and nuclear screening of all cargos coming into the United States.

At her first hearing before the House of Representatives Homeland Security Committee, Napolitano said the 2012 deadline set by Congress is not going to work.

"To do 100 percent screening requires agreements with many countries," Napolitano said, echoing a position taken by officials in the former Bush administration.

A law passed by Congress in 2007 requires the Homeland Security Department to screen all cargo headed for the United States by 2012. About 11.5 million containers come into the country each year.

Total screening also could significantly slow commerce at busy ports, and at least 27 countries and major industry associations have found significant problems with how they would be affected by the law.

Among the major obstacles to meeting the deadline is deploying trained U.S. officials to more than 700 foreign ports to operate scanning equipment.

Napolitano said the agency currently screens almost all cargo containers considered suspicious. She has said she agrees with the concept of catching threats before they reach the United States.