Zeljko "Steven" Mitrovic, 45, was shot dead on Tuesday at his Dynamic Transport business at Wetherill Park, in Sydney's southwest.
Since the shooting, police have been unable to locate Frank Bacic, who they named in the hours after the killing as a "priority witness". Police believe Mr Bacic, 38, is either "laying low, dead or overseas".
Officers think Mr Mitrovic may have been acting for an associate who owed money to Mr Bacic.
Mr Bacic allegedly had debts of his own to another large group of men who had savagely beaten him in order to recover their money. It is believed he was under serious pressure to repay his debt and urgently needed the money from Mr Mitrovic's associate.
When the group came to collect money at Dynamic Transport, Bacic directed them to Mr Mitrovic, saying he could settle the debt.
Seven men, thought to have included Mr Bacic, were captured on CCTV cameras having a heated argument in the company yard. As Mr Mitrovic left the area to his first-floor office, three of the men followed him upstairs where another heated altercation took place.
According to one underworld source, Mr Mitrovic told the group to "f ... off" and, as a consequence, was shot.
"He was a bit brash like that, but that was the way he was," the source said.
Police said Mr Bacic was now being sought for assistance. There is no suggestion that Mr Bacic ordered the attack or was involved in the shooting.
"We can't find him, he's either laying low, dead or overseas," a source said.
Other areas of Mr Mitrovic's life are also being investigated, including a caveat placed on his Kenthurst home by City Nominees, a company run by the family of Mr Mitrovic's former business partner, George Alex, 42. City Nominees withdrew the caveat on December 21.
Police are preparing for Mr Mitrovic's funeral service tomorrow, which is expected to draw a number of outlawed motorcycle members from interstate.
Mr Mitrovic lived in NSW but was a member of the Hells Angels newly-formed Burleigh Heads chapter in Queensland. A "run" involving a number of Hells Angels bikers is expected to take place along the M5 in his honour.