Kevin Carroll, 29, who was known as Kevin the Gerbil, was shot dead while he sat in the back of a black Audi car outside the Asda store in Robroyston, Glasgow, on January 13.
He was known to have links with the criminal underworld and survived an attempt on his life three years before.
Police said three masked men sped from the scene at about 1.30pm in a dark blue Volkswagen Golf with the registration number YF55 EZZ.
Detectives have urged people who feel uncomfortable speaking to police to e-mail information to
Detective Supt Michael Orr said: "I know that there are individuals with criminal connections who will know about this murder. I am keen to hear from anyone who has heard anything about this case."
He said the content of people's e-mails would be treated with the "utmost confidence".