The documents provide more details about the surveillance capabilities of the department’s unmanned Predator B drones, which are primarily used to patrol the United States’ northern and southern borders but have been pressed into service on behalf of a growing number of law enforcement agencies including the FBI, the Secret Service, the Texas Rangers, and local police.

Homeland Security’s specifications for its drones, built by San Diego-based General Atomics Aeronautical Systems, say they “shall be capable of identifying a standing human being at night as likely armed or not,” meaning carrying a shotgun or rifle. They also specify “signals interception” technology that can capture communications in the frequency ranges used by mobile phones, and “direction finding” technology that can identify the locations of mobile devices or two-way radios…

The prospect of identifying armed Americans concerns Second Amendment advocates, who say that technology billed as securing the United States’ land and maritime borders should not be used domestically. Michael Kostelnik, the Homeland Security official who created the program, told Congress that the drone fleet would be available to “respond to emergency missions across the country,” and a Predator drone was dispatched to the tiny town of Lakota, N.D., to aid local police in a dispute that began with reimbursement for feeding six cows. The defendant, arrested with the help of Predator surveillance, lost a preliminary bid to dismiss the charges.