LOS ANGELES » The arrest in the case of the "Grim Sleeper"—a serial killer who terrorized South Los Angeles for two decades—has put one of the hottest controversies in U.S. law enforcement to its first major test.

Only two states, Colorado and California, have a codified policy permitting the use of familial search, the use of DNA samples taken from convicted criminals to track down relatives who may have committed a crime. It is a practice that district attorneys and the police say is an essential tool in catching otherwise elusive criminals, but that privacy experts criticize as a threat to civil liberties.

This week, law enforcement struck a significant blow for the practice when the Los Angeles Police Department used it to arrest a man who they say murdered at least 10 residents here over 25 years. It is the first time an active familial search has been used to solve a homicide case in the United States.

Lonnie D. Franklin Jr., 57, was charged Thursday with 10 counts of murder and one of attempted murder after the state DNA lab discovered a DNA link between evidence from the old crime scenes and that of Franklin’s son, Christopher, who was recently convicted of a felony weapons charge.

The information developed from the state’s familial search program suggested that Christopher Franklin was a relative of the source of the DNA from the old crime scenes. The police confirmed the association of Lonnie Franklin through matching of DNA from a discarded pizza slice. The match provided the crucial link in a seemingly unsolvable crime that struck terror and hopelessness throughout one of the city’s poorest areas for years.

Chief Charlie Beck of the Los Angeles Police said Thursday that he expected to connect Lonnie Franklin, who is being held without bail, to other murders.

"This is truly a breakthrough," said Attorney General Jerry Brown, whose office wrote the DNA policy, in a telephone interview. The use of the practice demonstrates that law enforcement can "stop criminals in their tracks and lock up some of the most vicious and dangerous members of our society," Brown said. "That’s why this technology is so important."

The arrest in the protracted, gory case could settle the internal debate among lawmakers and the law enforcement agencies across the country considering the use of familial search, evidence law experts said. California is awaiting a court ruling on whether its DNA database can be expanded to include arrestees.

The Los Angeles case "shows why it can be tremendously useful in cases that seem pretty dead and hard to crack," said Jennifer Mnookin, a law professor and evidence expert at the University of California, Los Angeles. "So therefore we will very likely see an increasing use of these techniques, and at a minimum one hopes it is done in a sensitive way that is thoughtful and attentive to the concerns" of its critics, she said.

At least some of those critics remain skeptical.

"Familial searching is a tool, and at this point it is a very imprecise tool," said Michael Risher, a lawyer with the American Civil Liberties Union of Northern California, who added there was the possibility of innocent people being harassed in the pursuit of a crime.

Many law enforcement agencies collect DNA samples of convicted felons in hopes that DNA from other crime scenes can be matched to those individuals.

In the case of a close but imperfect match between crime scene DNA and that of someone locked up, forensic scientists say, the person responsible in that crime may well be a relative of the person locked up. Through a software tool, scientists are then able to painstakingly parse the DNA to determine sibling or parent-child relationships, information the police use to pursue possible suspects.

While the practice is common in England, it has been limited largely to Colorado in the United States. But in 2008, the California Department of Justice began using familial searches—in the face of significant protests—to solve hard crimes. The state restricted the practice to major, violent crimes in which all other investigative techniques had proved fruitless.

Those who oppose the technique argue that there are inherent privacy concerns, and that it serves, in essence, as a form of racial profiling because a higher proportion of inmates are members of minorities.

"I can imagine lots of African-American families would think it is not fair to put a disproportionate number of black families under permanent genetic surveillance," said Jeffrey Rosen, a law professor at George Washington University who has written about this issue.

Further, Rosen said, if other jurisdictions were not as strict at California about the technique’s application—expanding it to nonviolent crimes, for instance—the issue would be even more complex.

"The technique is not inherently good or evil," he said. "It all has to do with what crimes it is used for, who’s in the database, how the database is regulated and what is done with the samples."

Investigators here had tried for years to solve the case of the Grim Sleeper, so known because of a 13-year hiatus in the killing streak, who committed the majority of his killings in the 1980s but in recent years had restarted his spree. The department, in conjunction with the state attorney general’s office, ran a familial DNA search for the first time in 2008 that was unsuccessful, said Det. Dennis Kilcoyne, who led the investigation, at a news conference Thursday.

About a year and a half later there was the second run through the system, the detective said, and the department "learned of a man that as we know now turned out to be a direct relative of our suspect." The authorities were then able to narrow their focus to the elder Franklin over the past weekend based on the proximity of his residence to the crime scenes, race, age and other factors.

Police then conducted around-the-clock surveillance of Franklin, following him as he walked or went on drives, and retrieved a plate and napkin he had thrown away after eating pizza, which provided the DNA match.

Only after closely guarded procedures were the victims’ families alerted and Franklin arrested Wednesday as he went to move his car.

The Grim Sleeper began his killing of women (and one man) in South Los Angeles in 1985, shooting his victims—some of whom were prostitutes—and leaving them in alleyways and dumpsters. The killer stopped in 1988, then started again in 2002.

Gurtha Cole, 80, lives with her 38-year-old granddaughter in a building across the street from the home of Franklin, who the police say worked as a mechanic for the city’s Department of Transportation. She expressed surprise.

"He’s very social with us," she said. "On holidays and stuff he would invite the neighbors to come over to a picnic and host everyone."

At a news conference in downtown Los Angeles, a smattering of victims’ relatives squeezed together into the crowd, wearing solemn expressions. LaVerne Peters held a photo of her daughter Janecia Peters—the most recent victim, killed in 2007—wearing a green cap and gown.

"This presents some relief," Peters said. "There is nothing worse than having a child murdered."