By JENNIFER STEINHAUER

LOS ANGELES — Jamiel Shaw Sr. never gave much thought to the immigration status of gang members in his South Los Angeles neighborhood. With his military wife deployed to Iraq and two sons to raise, there were football practices to manage, shoes to buy, college applications to consider.

But in the two months since his older son, Jamiel Jr., was gunned down by a man the police say is a gang member who was here illegally from Mexico, Mr. Shaw has been able to think of little else.

“I don’t care about illegal people who are working here and taking care of themselves,” Mr. Shaw said. “I just feel I am obligated to target illegal aliens in gangs.”

A preliminary hearing in the killing of Jamiel Shaw Jr. is set to begin here on Thursday. Jamiel Jr. — who was black and, according to the police, not known to be affiliated with gangs — and a simmering unease about illegal immigration have unleashed a swell of opposition to the city’s hands-off policy toward immigration enforcement.

The Los Angeles Police Department was one of the first in the nation — nearly three decades ago — to institute a procedure that prohibits officers from initiating contact with people for the sole purpose of learning their immigration status. The procedure, known as Special Order 40, was designed in part to reassure illegal immigrants who historically had shied from reporting crimes and assisting police investigations.

But in the context of contemporary immigration politics, the procedure is now perceived in black neighborhoods and beyond as a roadblock to using immigration laws as a tool against Latino gang violence. A push to reverse the procedure, led by Mr. Shaw and viewed by many as a symbol of deeper racial conflicts in South Los Angeles, has inflamed tensions between many blacks and Hispanic immigrants, groups long resentful of each other as shifting demographics and a smattering of racially motivated killings have racked South Los Angeles.

“I think you can assume the resentments are pretty widespread,” said Connie Rice, a civil rights activist and lawyer. “There has been a huge turnover in a 20-year period, and so the tensions get expressed in a lot of other ways. The African-American community is feeling under siege, and it is always easier to strike out at the ‘other.’ “

Councilman Dennis Zine, a former police officer, has proposed amending Special Order 40 to require that gang members found to be illegal immigrants be reported to federal authorities. Judicial Watch, a conservative legal group based in Washington, has filed a suit in federal court seeking to do away with the order, arguing that it unconstitutionally impedes cooperation between the local police and federal immigration agents.

And Mr. Shaw and other grass-roots activists have called upon the police chief, William J. Bratton, to intervene.

“All we are saying is that when an officer encounters an individual who is in the gang file, they would check the name through the system,” Mr. Zine said. “If they are a known gang member, they would then inquire of their immigration status.”

Over the last year, law enforcement agencies across the country have begun to revisit policies like Special Order 40 after public pressure over crimes by illegal immigrants. Most notably, the sheriff of Maricopa County, Ariz., has conducted immigration sweeps, and last summer, the New Jersey attorney general told law enforcement agencies to begin asking about the immigration status of criminal suspects.

But the assault on Special Order 40 has brought sharp criticism from some immigrant advocacy groups and a rebuke from Chief Bratton, who said he viewed the alternative as institutionalized racial profiling.

Nothing in the order prevents law enforcement officials from asking people about their immigration status when arrested, or reporting people charged with crimes to immigration officials; but the order prohibits the police from pursing illegal immigrants for their illegal status alone.

“We make very concerted efforts to deal with the criminal element in the immigrant community,” Mr. Bratton said. “But I’m not going to aggressively pursue people whose only crime is to come into this country illegally.”

Further, Mr. Bratton said, “stopping everyone who looks like an immigrant to inquire about their gang status would only inflame racial tensions.”

“This is a city that’s extremely race conscious,” he said.

Many police officials and others believe that small pockets of racial tension are being exploited by anti-immigrant groups.

“What we largely see are people living together, going to school together, intermarrying and living together just fine,” said Deputy Chief Sergio Diaz, who oversees police operations in many gang-infested Los Angeles neighborhoods. “Why would we let gang members define race relations in this city?”

The police say the man charged with murder in the killing of 17-year-old Jamiel Shaw Jr. had entered the United States illegally and was a member of the notorious 18th Street gang. The man, Pedro Espinoza, 19, had recently been released from jail after being convicted of weapons possession and resisting arrest.

The day after his release, Mr. Espinoza made his way to the Shaws’ street in the mid-city section of Los Angeles, rival gang territory.

The police, citing the investigation, would not say why Mr. Espinoza went there, but they did say he approached Jamiel, a high school football star who was making his way home from the mall, and asked him what gang he belonged to, a well-known code among gang members that shots are coming.

Before Jamiel could answer, the police say, Mr. Espinoza shot him twice, in the chest and head, and he died on the sidewalk.

The police say that they have no reason to believe that Jamiel had ever been in a gang, but that his accused assailant “thought his victim was a rival gang member,” said Detective Frank Carrillo.

But suspicions in many parts of Los Angeles that the case might not be so clear-cut have been fueled by a posting on myspace.com that appears to have been written by Jamiel. Alex Alonso, a Los Angeles blogger who writes extensively about gangs, has interpreted the posting to indicate that Jamiel was associated with gang members. For instance, the posting reads, “IM A TRU G,” which Mr. Alonso said means “true gangster.”

Jamiel’s family has strongly denied that he had any gang associations and has insisted that prosecutors pursue hate-crime charges against Mr. Espinoza, because, they say, their son was attacked because he was black.

Some gang experts say it is not uncommon for nongang members to associate with gang members in their neighborhoods.

“It is so easy to be mistaken for a gang member if you’ve grown up in an area where people you’ve known all your life are from this gang or that gang,” said Celeste Fremon, who has written books on Los Angeles gangs. Having childhood friends and associates who have gone on to join gangs could lead to “someone mistaking him for a gang member,” Ms. Fremon said.

Though the killing has brought Special Order 40 into stark relief, the procedure did not actually apply to Mr. Espinoza; his prior run-in with the police was in Culver City, a separate city with its own police force that does not use the order.

And even when the police alert federal immigration officials that they have an illegal immigrant in custody, the officials do not always act on it, the police said.

“He was not identified by immigration,” Deputy Chief Diaz said of Mr. Espinoza. “But I suspect had he been identified, nothing would have happened.”

In recent years, the Los Angeles police and county sheriff’s departments have worked more closely with federal immigration officials on the problem. The number of inmates in Los Angeles County jails who were interviewed by Immigration and Customs Enforcement officials more than doubled to 8,754 in the fiscal year 2007, from 4,079 in 2004. Last year, 4,815 of the interviewed inmates were detained — held without bail and flagged for possible deportation while their charges played out. In 2004, 3,121 inmates were detained.

“Like any law enforcement agency, we have finite resources and we have to prioritize things,” said Virginia Kice, a customs spokeswoman. “But given the import of gang activity here in L.A., if we have information that someone is an active member of a street gang, that is a priority.”

Gang violence has formed some negative racial perceptions among blacks and Latinos, even though 87 percent of the 394 killings in Los Angeles last year were by people of the same race as the victim, according to police records.

Antonio Cabrera, who pushes a small ice cream cart near the park where Jamiel went to high school, said he had been robbed three times by black gang members.

“The truth is, that makes me think that other blacks are going to bother me,” Mr. Cabrera said, though he added that he generally viewed gang members of all races as equaling threatening.

In recent days, Mr. Shaw has found himself able to recount, somewhat matter of factly, the ordeal of hearing gunshots, opening his front door and seeing his son on the sidewalk in a pool of blood, bits of his head scattered near his fallen iPod.

But then he opens his car trunk and sees the half-empty bottle of cranberry juice that his son discarded before a recent football practice, and Mr. Shaw finds his breath gone, consumed by the tugging agony of loss.

“Every day I just see that boy laying in the street dead,” he said, sobbing. “I just want to get him back. That’s why I can’t stop. I’m on a mission. I can’t stop.”

Ana Facio Contreras contributed reporting.