PHOENIX - Homeland Security Secretary Janet Napolitano said Tuesday that she will freeze funds for expanding the virtual fence that originally was supposed to monitor most of the 2,000-mile southern U.S. border by 2011 but now covers only a portion of Arizona's boundary with Mexico.

A sign on the fence of a ranch along the U.S./Mexico border that discourages all trespassers but the Border Patrol near Campo, California in this photo taken March 17, 2008. (REUTERS/Fred Greave) The virtual fence is a network of cameras, ground sensors and radars designed to let a small number of dispatchers watch the border on a computer monitor, zoom in with cameras to see people crossing, and decide whether to send Border Patrol agents to the scene.

A string of technical glitches and delays has put the virtual fence in jeopardy. Two months ago, Napolitano ordered a reassessment of the project that has thus far cost the government $672 million.

"Not only do we have an obligation to secure our borders, we have a responsibility to do so in the most cost effective way possible," Napolitano said in a statement, which didn't specify the amount of funding that would be frozen. The funds will be frozen until the project's reassessment is completed.

Napolitano also plans to redirect $50 million from the Arizona portion to pay for radios, cameras, thermal-imaging devices and other technology that would be used at the border but wouldn't be strung together in the vastly networked way envisioned for the virtual fence.

U.S. Sen. John McCain, a Republican from Arizona, applauded that decision, saying he was pleased that "Napolitano has decided to instead turn to commercial available technology that can be used to immediately secure our border from illegal entries. I have been calling for congressional oversight and administrative action on this issue since it became clear that SBInet was a complete failure."

The fence is known within the government as SBInet.

U.S. Rep. Bennie Thompson, who leads the House Homeland Security Committee, called the project an expensive disappointment.

"Today's announcement is a recognition that this troubled program needs better management and stronger oversight," the Mississippi Democrat said in a statement.

Among other problems, the radar system had trouble distinguishing between vegetation and people when the weather was windy. The satellite communication system took too long to relay information in the field to a command center; by the time an operator moved a camera to take a closer look at a spot, whatever had raised suspicion was gone.

The first permanent segment of the virtual fence - a 23-mile stretch near Sasabe, Ariz. - was supposed to be turned over to the Border Patrol by the main contractor, Boeing Co., for testing in January. The handover was delayed by problems involving video recording equipment. Testing is continuing on that section of virtual fence.

Construction on a second 30-mile permanent section south of Ajo, Ariz., is still expected to be completed in August.

A 28-mile prototype virtual fence has been in use in southern Arizona for two years, but that stretch will be replaced by the first permanent section.

The virtual fence was developed as part of then-President George W. Bush's border security plan. It was meant to add another layer of protection at the border, along with thousands of Border Patrol agents and 650 miles of real fences.

Originally, the virtual fence was supposed to be completed by 2011; that date has slipped to 2014, largely because of technical problems.

Both Boeing and the government officials said the technical problems stemmed from an erroneous belief that the first-of-its-kind virtual fence could be put together relatively quickly by tying together off-the-shelf components that weren't designed to be linked.

Homeland Security officials have said the government shares blame with the contractor for the delays in the virtual fence.