The Met Police force is offering a £20,000 reward for information relating to the killing of Jazmel Patterson-Low in south London last year.
The 26-year-old was shot in Lambeth and died half an hour after a driver dropped him off at hospital in the early hours of 11 May 2024.
A post-mortem examination gave his cause of death as a single gunshot wound.
A year later, detectives said they were still searching for those involved.
A force spokesperson said people coming forward with information would be "supported throughout" and identities could be protected.
He added: "Alliances change over time, so please do the right thing by coming forward.
"We know there was a group of around 40 people gathered there on Friday evening and into the early hours of Saturday morning. If you were there, then you may have crucial information for us."
A 25-year-old man arrested shortly after the shooting on suspicion of murder was later released without charge.
