US Park Police Sgt. Walter Sherba peers at a monitor glowing in his car's dark interior, his face bathed in spectral sheen. "Someone complained that kids were having a party," says Sergeant Sherba. Nothing is visible through the car's night-shrouded windscreen. But etched on the monitor are the deserted woods of a Washington park. It's a false alarm, he says, but "if someone was in front of us, they would really glow."

The infrared scanner mounted on his car is the same one used by US troops to hunt Iraqi forces in the Gulf war. It is symbolic of an increasing use by police of some of the advanced technologies that make the US military the world's mightiest.

But civil-liberties defenders warn that such high-tech tools could increasingly be used by police to invade personal privacy. "In terms of civil liberties implications, bells should be going off," says David Harris, a professor at the University of Toledo Law School in Toledo, Ohio.

As the use of these technologies increases, courts will likely confront new cases requiring them to reexamine the legal barriers designed to protect Americans' constitutional privacy rights.

But police say that because of the increasing sophistication and violence of criminals they need such tools.

*Dallas police have tested a system called SECURES that pinpoints the location of gunfire. With microphones mounted on buildings and street lights in high-crime areas, SECURES detects the source and lights up a map at police headquarters. The system, was developed by Minneapolis-based Alliant Techsystems, which makes submarine-detection systems for the Navy.

A variation designed by the Lawrence Livermore National Laboratory in Livermore, Calif. - a nuclear weapons research center - actually tracks bullets in flight, pinpoints the shooter's location, and can return fire.

*In Redondo Beach, Calif., police have tested a system that uses the military's Global Positioning Satellite network to keep track of its patrol cars. It even tells headquarters when officers turn on their sirens.

Hughes Aircraft Company, a defense contractor based in Arlington, Va., is going a step further: It is trying to interest police in its systems for the 21st-century soldier. These include a helmet-mounted video display in which the officer sees either a night-vision display or maps beamed down from satellites; mounted on a gun are a camera, a night-vision scope, and a laser sight.

*Lockheed-Martin Corp, the aerospace giant based in Bethesda, Md., and Deerfield, Mass.-based Millitech Corp. are developing systems that are essentially portable versions of airport metal detectors. By measuring differences in electromagnetic radiation emitted from the human body and solid objects these systems allow police to scan people for concealed weapons or other contraband - even without them knowing.

High-tech = high cost

The main reason new technologies aren't in widespread use is their price tags. Most of the nation's 17,300 police and sheriff departments prefer to devote any extra funds to hiring additional officers.

But making such technologies available and affordable is one facet of the Clinton administration's anticrime efforts.

Under a five-year, $37.5 million program, the Justice Department, working with the Pentagon, began in 1994 to fund adaptation of military systems for law enforcement.

The work is going on at both military labs and research centers that design the US nuclear arsenal. The government is also awarding grants to defense firms eager to find new markets to stem slumping post-cold-war weapons sales.

Thermal guns

The impetus for the program came with the end of the cold war, which saw US troops embarking on peacekeeping operations that included tasks similar to those faced by civilian police.

By underwriting such "dual use" technologies, the administration decided it could bolster the war on crime, while saving money and preserving defense jobs.

Some technologies being explored are: "smart" guns that can be fired only by their owners, thereby protecting officers from being shot with their own weapons; lighter and stronger body armor; guns that fire nets to capture or subdue suspects; a thermal gun that disables a suspect by temporarily boosting body temperature; and nausea-inducing strobe lights.

One tool that was designed to fire an electromagnetic pulse that would disable a fleeing vehicle by "frying" its electrical system had to be stopped because of liability considerations.

Seeing through walls

There's one class of new tool that has enormous potential for aiding police - and has civil-liberties experts most alarmed: systems that will allow police to conduct searches without subjects ever knowing.

Prototypes have been developed that can "see" any objects - legal or illegal - through clothing.

In addition to replacing metal detectors in airports and public buildings, these systems may also come in hand-held or vehicle-mounted versions than could be used to scan crowds or people from afar.

Work is also under way on systems that use infrared technology to produce precise images of what's happening on the other side of walls.

"If I have a hostage barricaded in a room and I have to figure out a way to rescue the hostage and capture the hostage-taker, I'd like to know what's happening in that room," says David Boyd, director of the Office of Science and Technology at the Justice Department's National Institute of Justice.

But without some controls on the use of such instruments, some legal experts fear citizens could face an erosion of their constitutional right to privacy.

"As good as these machines are now, they are only going to get better and more stealthy and better able to do things without you knowing it," says Toledo law school's Dr. Harris. "It should concern everybody who wants to live in a society where people have a zone of privacy."

Advocates of spinning off military systems argue that while the majority of US police are using technologies that are decades old, they are increasingly finding themselves facing heavily armed criminals in combat-like clashes.

They cite the Feb. 28 battle between police and two body-armor-clad gunmen that turned a part of Los Angeles into a war zone.

"We have a battle going on in the streets of the United States every day," asserts Capt. Gary Van Horn, a US Park Police officer in Washington. "Let's give the police the technology we are giving the military. Eventually, courts are going to make a decision as to what is intrusive."

Judges have generally held that if searches are not physically intrusive and are accurate - such as those conducted by drug-sniffing dogs - they do not have to meet the legal requirements of "probable cause" or "reasonable suspicion."

Such searches, therefore, do not require warrants or violate the Fourth Amendment protection against unreasonable searches.

Relying partly on those criteria, several courts have ruled in favor of police who ferreted out indoor marijuana gardens by beaming infrared devices at homes to see the excess heat emitted by lamps used for growing the plant. Other courts have declared such searches illegal.

The US Supreme Court appears to be leaning toward allowing the searches, declining last year to hear a challenge to the legality of the technique.

The questions that these kind of devices raise were illustrated by the system mounted in Sherba's vehicle.

During an overnight shift last month, Sherba parked near two parks and scanned the undergrowth for muggers who frequently ambush gay men who rendezvous in the locations.

While no attackers were evident, the scanner clearly showed the bright outlines of liaisons amongst the trees.

Blending of purposes

Another concern of legal experts is that giving military technologies to law-enforcement officials will further blur the legal divide between soldiers and civilian police.

"You have a blending of purposes here," warns Mark Keppelhoff of the American Civil Liberties Union.

COMING TO A STREET CORNER NEAR YOU?

* Remote-control aircraft: BAI Aerosystems Corp., which manufactures unmanned spy planes for the military, has a law-enforcement model, one of which was bought by the FBI after the siege at the Branch Davidian compound in Waco, Texas.

Known as the Javelin, the 15-pound remote-controlled aircraft can be carried in the trunk of a car and launched by hand. It carries video cameras that instantly relay pictures to ground controllers.

The firm has also worked on an unmanned aircraft designed for crowd control. In tests conducted by the US Marine Corps last year, the aircraft deployed debilitating rocket-powered whistles, pepper spray, tear gas, and objects that puncture car tires.

* Finding hidden gunmen: The Lawrence Livermore National Laboratory, in Livermore, Calif., one of the nation's nuclear-weapons research centers, has designed a system for use against concealed gunmen. Known as Lifeguard, it tracks bullets in flight, pinpoints the shooter's location, and then relays it to either a camera or to a weapon that returns fire.