Mohamud Muketar, 31, was killed on the corner of Condell and Napier streets near the Fitzroy Police Station last week. Police want to find a man captured in CCTV walking across the street just minutes before the fatal attack.
Detective Senior Sergeant Steve McIntyre said he would have heard him crying out for help.
“We know that the deceased screamed out for help at the time of the attack, we believe that man would have heard those screams,” he said.
He thinks it is possible the man may have seen the attacker as well.
Police also want to identify a second man who was captured on CCTV riding a bicycle around the area that night.
Extensive door-knocking has been carried out in the neighbourhood but police are yet to identify a third man seen sprinting from the area, through Condell Reserve, just minutes after the attack.
He was described as about 183 centimetres tall, with a slim build and straggly shoulder-length hair.
The man was wearing a dark top and pants, and carrying a dark bag.
Muketar was diligent, devout man
Police are focusing their efforts around streets north of the corner of Napier and Condell streets, and all streets to Brunswick and Smith Street. Detetive McIntyre said they were yet to work out a motive.
“He was a quiet, diligent man who had a small group of friends and was quite religious and devout,” he said.
“His father is taking this quite poorly at this point of time.
“Obviously it’s a shock for any father to lose their son, but our understanding is they are a very close family, a religious family and obviously it’s got an impact that we can’t rectify at this point of time.”
Mr Mukatar was raised by his father in a refugee camp after fleeing war-torn Somali. They settled in Australia five years ago.
A GoFundMe campaign page set up to help the family said the 31-year-old was “full of love and joy with the rest of his life in front of him”.
“His dad is distraught at the brutal murder of his beautiful son and will not be able to work for some time.” An information caravan has been set up at the scene and anyone who was in the area between 8:00pm and midnight on April 20 has been urged to contact police.