The source said the first 24 hours were crucial in any such major investigation as they piece together exactly what happened.
And the ex high-ranking officer said police would be knocking on the doors of Mr Welsh’s “criminal associates” as well as tracking his final movements.
“The first thing is there will have been a number of people in and about the incident who will be key witnesses,” he said.
“Was he shot in the street or was he just found in the street? These are the types of things we need to know. Obviously the police will be keeping an open mind.”
The ex-officer said detectives would be keen to talk to those who knew Mr Welsh in trying to crack the case.
“He was a guy you’d call a ken-speckled character. He had a lot of connections. He was some-time showbiz and a part-time actor, but at the same time he had a lot of criminal connections.
“When you live a life like that, you’re going to have a few enemies. The police will be going through the motions, trying to piece together what happened.”
“Scene of crime investigations are incredibly detailed these days and the body will have been left until intricate forensics are carried out,” said the ex-cop. “They won’t have moved the body until all that has been done.”
Any evidence will have been bagged up at the scene and taken to a major investigation command centre along with photographs, added the source.
A crack team of detectives will be working the case and poring over the evidence, led by detective superintendent Allan Burton and chief inspector David Robertson.
The Major Investigation Team will comprise top officers with experience of such high-profile cases, added the source.
“They’ll be looking at his latest movements, any threats, looking at his criminal associates,” he added.
“It’s mostly businesses in that street with a few flats. It’s a very upmarket and prosperous area.
“They’ll be trying to put together his movements over the last few hours. They’ll be making sure they don’t miss anything and don’t follow any red herrings.
“Most of these cases are done and dusted in the first 24 hours and I wouldn’t be surprised in that’s the case.”