The Pentagon Tuesday asked reporters not to reveal details of the aircraft that will be used, because if the sniper has a military background, he may be able to evade detection by the plane.
A senior Defense Department official told United Press International the Pentagon considered using a Predator unmanned surveillance aircraft but rejected this as too dangerous in an area with such heavily traveled skies.
"It is a bad idea to fly an air vehicle without a pilot aboard in the congested air space around D.C., (it) makes a very high risk of a mid-air collision," the official said.
Predators are in heavy use in Afghanistan and elsewhere as they can linger over areas for long periods.
No matter what is done, officials say the Pentagon will make sure it does not violate the restrictions of the Posse Comitatus Act, which prohibits the military from being involved in law enforcement activities on U.S. soil.
The National Guard, when acting under the auspices of a state's governor, is excluded from the restrictions of the act.
A Pentagon official said the aircrew will provide only data to law enforcement officials and not identify "targets" for arrest.
The Defense Department has various counter-sniper systems that, employed on a battlefield or in an area known for snipers, can be used to track a bullet to its origin following a single shot. The systems rely on infrared sensors and acoustic sensors to determine the location of shooters.
One infrared camera system in use, known as the Viper, can be mounted on aircraft -- including remotely piloted unmanned aerial vehicles, planes and helicopters -- to survey wide areas. The Viper can be upgraded with an acoustic monitor to narrow target locations.
The Defense Department also developed two other systems, a fixed-site acoustic system called Secures/Tagit, which was used at the 1996 Olympic Games, and a fixed site acoustic system called Bullet Ears.
The systems can operate within a range of about a half-mile, according to the Defense Advanced Research Projects Agency.
The systems were developed under the direction of the Army's Training and Doctrine Command after 1993, when sniping claimed so many lives in Sarajevo during the Bosnian conflict.
DARPA experimented with four counter-sniper systems in 1996, and the military services procured limited numbers of the Viper, Bullet Ears and Secures/Tagit as a result of that work.
The Washington-area sniper has shot 11 people, killing nine, including a 47-year-old woman employed by the FBI who was shot Monday night in a Home Depot store parking lot in Falls Church, Va.
Police Tuesday released a composite drawing of two vehicles that could be linked to the shooting deaths, shortly after confirming that the death of a Virginia woman was the ninth killing linked to the sniper.
The description of the vehicles -- one a white Chevrolet Astro van and the other a white Ford Econovan -- was provided by "a number of witnesses" to the latest killing, this one located about 10 miles west of the capital, said Montgomery County Police Chief Charles Moose.
According to police, Monday's shooting happened in view of more witnesses than the previous attacks and some bystanders reported seeing the driver of the van. However, authorities said they did not have enough information about the suspect or the suspects to release a description to the public.
"When we feel good about it, we'll put it out there," Moose said. "But partial is just that, it's just partial, and often times that's just too weak to put out there."
Linda Franklin, 47, of Arlington -- a cancer survivor -- was shot once as she stood near her car in a Home Depot covered garage.
Franklin worked for the Federal Bureau of Investigation but was not assigned to the sniper case. FBI officials said she worked as an analyst for the counter-terrorism desk at FBI headquarters.
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