Because they escaped on foot and did not use a getaway car, police are convinced they received local assistance.
The St James' area of west Belfast where the shooting occurred is a warren of adjoining streets which would confuse strangers.
A major line of inquiry is that the killers were provided with a safe house by the gang that murdered their victim's father Tommy, an ex-Continuity IRA chief, and instructed to go there after the murder.
His son Warren was a dead man walking, having been accused of setting up crime boss Lawlor (right), who was shot dead in the Ardoyne area of Belfast in April while collecting a drug debt. His associates swore revenge on the 28-year-old cocaine dealer, and are understood to have contracted the same dissident gang that killed his dad to help with the hit.
Police at the scene of the murder of Warren Crossan on Saturday
Tommy Crossan was gunned down at his west Belfast fuel business in April 2014. Prior to his death, the 43-year-old had been in a bitter dispute with a former Provo-turned-dissident who is heavily involved in the illegal diesel trade.
This individual leads a crime gang that specialises in extortion and murdered Kieran McManus in 2013 and his friend Stephen Carson in 2016.
Last year cousins Michael 'Spud' Smith (41) and David 'Dee' Smith (35) were jailed for life for the killings.
Members of the same gang are now believed to have been involved in the fatal shooting of Warren Crossan, who was targeted getting out of a car at his mother Ann's home. He was blasted six times as he fled along St Katharine's Road while pleading for his life.
"The ex-Provo who had Tommy Crossan murdered is the main suspect. He is absolutely ruthless and would think nothing of taking a life."
Our source added: "It was always in the back of this fella's head that Warren would try and kill him in revenge for having his da shot dead. This was his way of getting in first to make sure that didn't happen."
Detective Chief Inspector Darren McCartney, who is leading the Warren Crossan murder inquiry, described the killing as "callous and reckless".
He said: "To bring firearms on to a residential street in broad daylight simply beggars belief. The gunmen did not give any thought to the risk posed to local people in this community when they ran through the streets firing shots, at least one of which struck a vehicle owned by a resident."
At the time of his murder Warren Crossan was on court bail accused of possessing £180,000 of cocaine with intent to supply. He was also on the run from gardai, having skipped bail for a series of burglaries in Donegal.
Notorious Dublin hitman Robbie Lawlor, who was shot dead in Ardoyne five months ago, was believed to be in Belfast to collect cash from Crossan relating to the cocaine.
His associates accused Crossan of setting him up to be killed, rather than pay the £180,000 debt.
The day after Lawlor was gunned down, two women were stopped by gardai with £50,000 travelling to Limerick, a city to which Crossan has strong family ties. Part of this cash is believed to have been payment to him for organising the murder.
Lawlor was known to have a bounty on his head for his role in the mutilation and dismembering of Drogheda teenager Keane Mulready-Woods in January.
In July, arsonists connected to the slain hitman petrol-bombed Warren Crossan's old home in the Co Antrim village of Crumlin.
The attack was carried out by friends of Robbie Lawlor who have placed a friend of his, Adrian 'Aidy' Holland, on a death list.
The hitman was shot dead in the front garden of the 36-year-old's Ardoyne home the day after he met him at a Co Antrim supermarket to exchange cash.
Giving evidence at a bail hearing, a detective explained Holland's links to Lawlor.
He revealed how arrangements were made for a second meeting the following day at a house on Etna Drive in north Belfast.
"Robert Lawlor did attend that appointment, and when he arrived he was met by a gunman who exited (the address) and shot him dead in broad daylight," said the detective.
"This murder is directly linked to a feud between several organised crime gangs originating out of the Republic of Ireland. It's a comprehensive and complex investigation, with numerous links to organised crime gangs operating internationally, as well as in mainland UK and throughout Ireland."
Holland has been lying low since the Lawlor killing, terrified of being murdered like his friend Warren Crossan.
Sunday Life understands that such is his concern he even avoided his pal's funeral.
Addressing mourners at the service, Fr Martin Magill said: "The two gunmen who got up that morning with the intention of executing Warren, their deliberate, callous way of gunning him down in daylight, in the presence of neighbours on St Katharine's Road, including children who witnessed it, demonstrated the worst of what human beings can do."