Army planes with high-tech surveillance equipment were preparing Wednesday to take to the skies around the nation's capital to help track a sniper who has eluded law enforcement officials for two weeks.

The planes were being flown to the region and were expected to join the hunt within days, a defense official said Wednesday, speaking on condition of anonymity.

Authorities called in the military Tuesday to help solve the baffling case that has left nine people dead and terrorized the capital area, leaving people afraid to go out of their homes. Defense Secretary Donald H. Rumsfeld on Tuesday evening agreed to an FBI request for the help, approving use of the Army's Airborne Reconnaissance Low plane, which has surveillance capabilities beyond those of local police forces, defense officials said.

The plan calls for military pilots to fly reconnaissance flights accompanied by federal agents, who would relay any collected information to authorities on the ground, a senior defense official said, speaking on condition of anonymity. A main objective is to improve communications among investigators.

The military planes join a chase that already is using officers from more than a dozen law enforcement agencies, along with dragnets, roadblocks, bloodhounds, helicopters and other tactics. Among important features of the four-engine plane are that it provides high-resolution imagery and night vision and looks like a small commercial plane, making it easier to blend in with local air traffic and avoid detection.

The Pentagon help will be given in a way meant to comply with the Posse Comitatus Act — a 19th century law that restricts the military's involvement in domestic law enforcement, said Pentagon spokesman Lt. Cmdr Jeff Davis. That means the military will not be involved in action on the ground, will relay data to law enforcement and not decide on its own what targets to watch, officials said.

The move is highly unusual but not unheard of.

During the last Olympic Games in Salt Lake City, military helicopters flew federal law enforcement agents around so they could do surveillance from the air.

Using several of the Army aircraft for possible 24-hour coverage, pilots would perform general reconnaissance, such as looking for or tracking the light-colored van that authorities say was seen at one or more of the shooting sites. Infrared sensors that can detect flashes of gunfire on the ground also could be used, officials said.

An unknown sniper or snipers has launched a series of 11 random rifle attacks in 13 days that has killed nine people and seriously wounded two others. All but one of the attacks have been in neighboring suburbs in Maryland and Virginia. One was just inside Washington at the Maryland border.

In two recent killings, police threw up a dragnet near the shooting site, blocking off streets and expressway ramps and stopping traffic to check vehicles. The assailant slipped away.

Another official, who also discussed the matter on condition of anonymity, said that at the request of investigators, the Army has started searching its records for people trained as snipers for any former or current service member who might be involved in the shootings.

Law enforcement officials have not said they suspect anyone from the services. Experts have said the shooter also could be a hunter, a target shooter or someone with law enforcement experience.

Police from counties where the attacker has struck are participating in the joint investigation as well as both state police forces, Washington's metropolitan police, the FBI and the federal Bureau of Alcohol, Tobacco and Firearms.

Meanwhile, federal investigators refused Tuesday to rule out the possibility that organized terrorist groups are behind the shootings.

"The communities are terrorized," said the homeland security director, Tom Ridge.