They may not try as hard in some cases where witnesses and the victims friends and family won't cooperate with LEO, but they do always open an investigation. I remember a drive-by shooting at an outdoor birthday party between 2 Salvadorian gangs where the attending family members, including the grandmother, wouldn't talk to the police even though two people died. I guess the family was waiting to retaliate themselves in due time. Sometimes a young gang-banger will be found in a yard of an abandoned house and LEO will never get any leads besides the file they may have already put together on the subject while he was alive. It's a different culture in these neighborhoods. There is a portion of those communities who don't want the police to interfere with their way of life, as messed up as it may be.