One major question still at issue is how much the virtual fence will ultimately cost.

One congressman has put the multiyear cost of SBInet at US$30 billion, while the Department of Homeland Security will only estimate the cost for the southern border with Mexico. SBInet is the high-tech portion of DHS's Secure Border Initiative announced in November 2005.

The DHS estimate for the southern border portion of SBInet is $7.6 billion through 2011, including fencing, ground sensors, radar, cameras and command, control and communications equipment, according to a report (pdf format) issued last week by the U.S. Government Accountability Office.

Milestones have not been set for the northern border, although DHS told the GAO that the upper limit of funding will provide border security technology for "6,000 miles of secure U.S. border," which is for both the northern and southern borders. The funding estimates provide primarily for technology in SBInet and do not include costs such as added border patrol agents in the broader SBI.

Two congressional Democratic leaders in recent hearings also have questioned the way the Boeing contract is managed. One hearing was held last week before the House Homeland Security Committee.