The cop was gruff, terse, as he pulled over the faded green Ford. "License, registration," snapped Sgt. W.J. Kleckley. The driver knew instantly why he'd been stopped. He was white, out of place in the ghetto, home to many of the 20 missing or murdered black children whose cases are no closer to resolution now than 19 months ago, when the mysterious killings began.

But the policeman didn't pull the driver over on a whim. Police, in their attempt to follow up leads that seemed to evaporate, have stopped dozens of white males in green cars as possible witnesses to circumstances surrounding the murder of a black child whose body was found two weeks ago in nearby De Kalb County.

A dental technician, under hypnosis, had described a white man with a mustache and sandy hair behind the wheel of a green car as the man she spotted near the woods where the killer is believed to have dumped the body three days earlier. Her description fit.

"God in heaven knows I wouldn't hurt a kid for nothin' in the world," said the edgy driver, Lawrence Montgomery, a $3.75-an-hour laborer at the landfill next door to a housing project who was dropping off a friend after work. "I got a little girl 7 months old . . . I'm a working man." He took a breath. "Hey," he asked. "Is it a white person doing it?"

"Don't know," said Kleckley, "but you can't take no chances. You got to be suspicious of everyone."

Eighteen black children have been killed, and two others have disappeared in Atlanta since July 1979 -- a murder rate vastly greater than the six children normally found dead each year in the entire city. Today, two additional boys, ages 11 and 14, were reported missing but later were found -- one at a friend's house and the other reportedly playing basketball.

Police are baffled about who is responsible, but most investigators are working on the assumption that there are at least three sets of killers.

The primary killer is believed to be black, a man in his 20s, working either alone or with another person -- a psychopath driven by a compulsion to kill young black boys, someone who is able to move about unnoticed like a phantom, gain his victim's trust and spirit them away.

He is believed to be responsible for at least nine killings. A copycat killer, inflamed by the publicity the murders have generated, is blamed for others. Police speculate that some may be drug-related, or possibly can be laid to a relative, or someone close to the children.

The victims have been black, all males except for two girls, all between 7 and 15. Most were street-smart youngsters who hustled for money outside shopping centers. They were hungry, ambitious, driven by the ghetto chutzpa to make a buck, to get ahead, and if lucky, to make something of themselves.

That survival trait made them vulnerable to a killer whose identity has baffled and frustrated authorities as much as the elusive white man in a green car.

Lead after lead dead-ends. But police agree, when, or if, the cases are solved, it will likely be because some beat cop like Kleckley spotted something suspicious.

So far, little tangible has come from 35 investigators of the Atlanta police, the special task force dispatched by President Reagan, the upwards of 25 FBI agents and scores of county police working night and day on the cases.

Atlanta Public Safety Commissioner Lee P. Brown admits that police are stumped. "Homicides are normally solved in one or more of three ways," he said, explaining what makes the child murders so perplexing. "You have an eyewitness or a confession or an abundance of physical evidence. We have none of these. A child disappears and a body is found. We don't have a crime scene to work with."

Police have taken extraordinary measures. In De Kalb County, where Patrick Baltazar was found murdered -- strangled by a rope and dumped in the woods -- police with a reputation for tough law enforcement have been stopping virtually all light-skinned black men driving cars with young boys inside, especially if the child is black and appears to be in distress. The result: Complaints about possible civil liberties violations.

Police insist they must be flexible. "How do you know his father wasn't scolding him?" asks one police officer. "You don't, but under the circumstances we'd be negligent if we didn't stop him. We're stopping anything suspicious."

In Atlanta, that includes vans, which some speculate could be used to carry bodies undetected about the city and whose carpet fibers could account for the similar fibrous material medical examiners have found on the bodies of six children.

Some police are studying hours of videotapes taken at the victims' funerals, scouring the crowds for a possible suspect who might revel in the aftermath of his work.

About 300 children dispatched to the Fulton County Juvenile Detention Center have been interviewed for leads because investigators believe the killer has tried and failed to abduct other children.

But one investigator says that the children have failed to provide anything solid. "If they were late in getting home, the best excuse is to say, 'A man was chasing me,'" he said.

Hypnosis and voice stress analyzers were used in interviews, and some parents have taken lie-detectgor tests. Police pore over detailed maps and charts that adorn the walls of task force headquarters, tracking the movements and relationships of the victims.

The FBI's behavioral sciences unit at Quantico, Va., has drawn up a profile, virtually out of thin air, of the possible killer or killers, and has resurrected unsolved murder cases -- such as the 1979 killngs of seven children in a Detroit suburb -- to search for similarities that might help them crack the killings here.

Like Atlanta, lthe Michigan victims in the "Woodward [Avenue] Corridor" cases were mostly boys who were found "in very good condition," said FBI criminologist Robert Ressler.

"They had their clothes cleaned from the time they left home and had been fed since their abduction, with the bodies positioned so they could be found," he said, a factor in the Atlanta killings that lead police to believe the killer wants recognition and notoriety and has a desire to be caught.

"Not that we think there is any connection, but looking at one might give you insights into looking into the other," he said. "From the Michigan case, we pretty firmly have an idea what kind of person would have done it."

From studying child murderers, Ressler said he believes "the type of killing does not cross racial lines," leading police to a near certitude that the the prime Atlanta killer or killers are black. "Their state of mind is highly abnormal and their conventional mental illness" has degenerated into "criminal acts."

In Atlanta, nine of the victims have been either strangled or asphyxiated and found miles away from where they were last seen. In some cases, police and volunteer searchers who have combed the woods every Saturday for the last five months have found only bones of badly decomposed bodies.

None of the victims has shown any signs of sexual molestation, but that does not rule out a sexually motivated killer, police say.

"Some psychopaths are sexually motivated but do not appear to be," said Ressler. "David Berkowitz [convicted in the "Son of Sam" killings] was sexually motivated, but his acting out was limited to shooting women with a .44-cal. gun at night. That's very Freudian, but not overtly sexual. Berkowitz felt he was sexually inadequate and couldn't bring himself to rape, but his crime was an acting out against women who rejected him.

"We've studied many killers who stopped short of sexual assault because of feelings of inadequacy, and that leaves it open in the Atlanta cases. It's a good bet that a sexual psychopath is one type of suspect we're looking for."

Epidemiologists at the Communicable Disease Center, which solved the mystery of the toxic shock syndrome, are using their expertise to prepare a profile of children who might be most vulnerable, that is, those who are black and poor and hustle on the streets to get ahead. Five of the victims had juvenile records.

Police have been deluged by more than 20,000 tips, telephoned in by citizens and stored and collated on giant computers, along with leads provided by beat officers like Kleckley.

More than 500 tips on green cars, for example, were phoned in to De Kalb police alone. Thirteen-hundred letters from psychics have been forwarded to Dr. William Rolls, a psychic expert in Chapel Hill, N.C., who discounts such information as an "unreliable and uncertain source of knowledge" that will not solve the cases.

FBI and state forensic scientists have attempted to take fingerprints from bodies of the victims, and crime labs have sifted soil samples from sites where the bodies were dumped on the outside chance the killer or killers may have left a clue. Police find he has covered his tracks well.