LOS ANGELES

--President Obama signed a sweeping aviation bill in February that will open American airspace to "unmanned aircraft systems," more commonly known as drones. Much of the recent discussion about the coming era of domestic drones, which will include those operated by companies and individuals, has been focused on privacy questions. However, drone proliferation also raises another issue that has received far less attention: the threat that they could be used to carry out terrorist attacks.

The technology exists to build drones that fit into a backpack and are equipped with a video camera and a warhead so they can be flown, cruise missile style, into a target. In fact, in September 2011 it was announced that the U.S. Army had signed a nearly $5 million contract with a California company, AeroVironment Inc., for the purchase of its Switchblade drones. A Switchblade launches from a tube roughly 2 feet long, sprouts wings immediately after exiting the tube, and is then controlled by an operator who looks into a shoe-box-shaped viewer displaying video from the drone. It is equipped with an electric motor that is quiet even when running, and that can be switched off to enable a completely silent glide in the final moments of approach.

Although reasonable people can disagree on how long it would take terrorists to build or acquire weaponized drones that can be guided by video into a target, there's really no dispute that it is a question of when and not if. The day will come when such drones are available to almost anyone who wants them badly enough.

There is ample evidence that terrorist groups have experimented with drones. As far back as the mid-1990s--practically ancient history in drone terms--the Japanese Aum Shinrikyo sect that carried out the sarin gas attack in the Tokyo subway reportedly considered drones. So too have al-Qaida and the Colombian insurgent group FARC.

Nations with a record of close ties to terrorists are another concern. Iran unveiled a drone in August 2010 that President Mahmoud Ahmadinejad managed to describe as an "ambassador of death" and a "message of peace and friendship" in the same sentence.