Ryan Chalmers, 21, suffered serious wounds after he was gunned down at close range by a man brandishing a shotgun in a park behind a bar in Coatbridge, North Lanarkshire.
Two years ago Mr Chalmers, of Airdrie, was acquitted of murder after a court accepted he acted in self-defence when he stabbed another man to death.
Detectives attended the scene of latest incident at around 6.45pm on Tuesday and sealed off an area near the Horseshoe Bar between Agnew Avenue and Deedes Street.
The gunman ran off and got into a dark coloured saloon car driven by an accomplice, which made off towards Airdrie.
Mr Chalmers was taken to nearby Monklands General Hospital, where medical staff described his condition as stable.
The suspect is described as in his early 20s with short, dark hair and wearing dark clothing.
Police searches have been carried out around the area of the shooting, but officers said they believed Mr Chalmers had been deliberately targeted.
Detective Inspector John Lamb said: "I know this incident will be concerning for many local residents, however I want to reassure them that this type of incident is rare and our inquiries so far lead us to believe this was a targeted attack.
"There will still be police activity in the area, with specialist search officers carrying out a detailed search. Officers are also carrying out extensive inquiries and studying CCTV footage in an effort to gather further information.
"If anyone has any information or indeed concerns, please speak to the officers on the ground.
"We will be using significant resources in this investigation and I would like to appeal to people in the area to contact police with any information.
"Any small piece of detail could prove significant and help us trace those responsible for the crime."
Police dogs and a forensic team were at the scene of the shooting, which remained closed off to the public as a uniformed police officer stood guard.
Officers could be seen searching in the bushes beside a path as police dogs worked alongside them. One local said: "Last night it was just the usual people who were in the pub here having a drink.
"But just before 7pm police cars turned up and cordoned off both entrances to the lane. People are saying he was taken down there and shot twice in the knee caps.
"They are saying this was payback for an incident he was involved in a couple of years ago and walked away from."
In August 2012, Mr Chalmers walked free from court after being charged with the murder of 42-year-old Christopher Bergin in December 2011.
The charge against him was found not proven by a jury at the High Court in Glasgow, who had heard Mr Bergin and two others burst into the flat where Mr Chalmers lived.
Mr Bergin's family spoke of their disappointment at the verdict.
Last year there was speculation that the case may be among a number under review by the Crown following changes to double jeopardy legislation.