The Homeland Security Department today halted expansion of its troubled virtual border fence project, citing cost overruns and missed deadlines.

In a statement, Homeland Security Secretary Janet Napolitano said the department will shift $50 million in stimulus funding from SBInet's initial phase in Arizona, near Tucson and Ajo, toward other proven, commercially available security technologies.

Homeland Security also has frozen all spending beyond the Tucson and Ajo deployments until an assessment of the project is completed, Napolitano said. The statement did not say when the assessment will be finished.

"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.

Spending on the patchwork of cameras, radar and sensors has soared since Homeland Security first awarded the contract for SBInet to Boeing in 2006. SBInet stands for Secure Border Initiative. The contract was originally for $2 billion and was expected to be finished by 2009. But the system has difficulty seeing clearly and often transmits false alarms.

SBInet now isn't expected to be finished until at least 2016, and the Government Accountability Office said in September that Homeland Security has already spent more than $3.7 billion on the project.

Napolitano is redirecting the stimulus money to buy mobile surveillance, thermal imaging and ultra-light detection devices, as well as x-ray units, mobile radios, cameras and laptops for pursuit vehicles.

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