Remember "In a play cast in hell there are no angels" and "There are no swans swimming in a cesspool." Don't be too disappointed by the actions of your witnesses, who will remain true to their gang culture by seeking street vengeance and refusing to help you.
Familiarize yourself with the victim's history and the history of the victim's gang and likely suspect gangs. Learn as much as you can about each gang's most active members. Don't be surprised if your victim's history is as bad as or worse than your suspects'. Remember the homicide code; he was a human being and deserves your best skills in doing your job.
No parent should have to bury their child, and often the victim leaves behind a wife and children. These family members bear little or no guilt for the crimes of the gang. Endeavor to be sympathetic and establish a working rapport with the victim's family. Often it will be these family members who will first hear rumors about who was involved and why. Your positive rapport may convince them to share these rumors with you.
Attend patrol briefings and give the patrol officers the general facts on the murder, remembering to exclude the agreed upon details. A personal appearance gives them a face and a person to identify with this investigation. Give them your contact information, offer some kind of small reward like for anyone bringing in significant leads or witnesses. I often offered a "six pack" bounty for information or an arrest when the suspect was identified.
Use these patrol officers, the gang or special projects units, station narco units, and any other support units to apply directed pressure on the target gangs. Keep these units interested and involved in the investigation. Be seen in the station often and continue your appearances at briefings.