Key entrances have been plugged in some of the biggest tunnels used to smuggle people and drugs across the U.S.-Mexico border but the passageways remain largely intact, raising concerns that smugglers might reuse them, according to a published report.
Critics say the unfilled tunnels pose an unnecessary national security risk.
Dozens of tunnels have been discovered underneath the border. Smaller passages are easily destroyed, but larger, more elaborate shafts require costly amounts of material and expertise to fill, authorities say. The task is further complicated if the tunnels run under private property.
Seven of the largest tunnels have yet to be filled in, including the so-called Grande Tunnel found in January 2006 that extends nearly a half-mile from San Diego to Tijuana, the Los Angeles Times reported Tuesday.
Filling them would cost about $2.7 million, Customs and Border Protection officials told the newspaper.
The agency is trying to find money in its budget to complete the work, said spokesman Michael Friel. The border agency's 2007 budget is $7.8 billion.
Sen. Dianne Feinstein , D-Calif., said the agency needs to try harder.
"The department should move, find money, and do it," Feinstein said. "This is a huge department with a huge budget. And if they don't have the money, they should tell us, and we will seek to get it in the emergency supplemental."
Smugglers may simply dig around plugged entrance points.
Traffickers have used one tunnel at Nogales, Ariz., three times in four years, said Agent Michael Cano, a Border Patrol spokesman.
Smugglers even take the concrete used to plug tunnel openings and use it to reinforce walls and ceilings in new tunnels, Cano said.

Copyright 2007 The Associated Press.