The report by the coroner, Keith Cutler, raises concerns about the Metropolitan police and the police watchdog over their handling of the shooting in August 2011 in Tottenham, north London, which triggered the worst riots in England in recent times.
The report contains recommendations aimed at preventing future deaths.
In January, an inquest jury decided Duggan was not holding a gun when shot by police but also found the decision by marksman “V53” to open fire was lawful.
The inquest heard that officers wrote up their accounts of the shooting over an eight-hour period, sitting together in a room. They were allowed to confer about the lead-up to the confrontation with Duggan, but not the actual incident itself.
In his report published on Wednesday, Cutler writes: “The fact of the officers gathering in a room together for many hours to compile statements created a perception of collusion.”
Cutler said a number of aspects of the way police wrote up their accounts “caused me concern”. This included omissions from initial accounts and delays in making statements abut what officers had witnessed.
The judge compares the fact that armed officers are given 48 hours before making their statements to the way a civilian witness is treated, saying: “It is not immediately obvious why a trained firearms officer should require what a civilian is not given.”
Conferring by officers after a shooting and the way they write their accounts has long been controversial. Police say it gives the most accurate evidence.
The Independent Police Complaints Commission wishes to stop officers conferring at all under new plans which police have criticised.
Duggan was shot after armed officers forced a cab in which he was travelling to stop, based on intelligence that he was part of a gang and had collected a gun.
Police had claimed that Duggan was shot twice after he brandished a firearm when surrounded by armed officers. The gun was found three to six metres (10-20ft) away, over a fence, and the jury decided Duggan discarded it as he exited the cab after police forced it to stop.
In a statement following the publication of the coroner’s concerns, the Met police said it had strengthened warnings given to officers about not conferring.
The Met said its firearms officers exercised huge restraint, opening fire only rarely, and said: “No officer sets out at the start of the day to run an operation that results in someone dying. But the task our officers face in making split-second decisions when confronting armed criminals and protecting the public means there is a risk – a very small risk – that this will happen. We want to ensure that we are doing the best job we can in difficult circumstances and we will now carefully consider and speak to other agencies about the concerns of the coroner.
“There is still an ongoing IPCC investigation and we await their findings and do not wish to be drawn into debating publicly the coroner’s concerns at this time.”
The coroner says law enforcement may have missed the chance to seize the weapons before Duggan collected one, and thus, it became necessary for armed police to challenge him. The shooting set off a series of events that rocked the country and saw several days of disorder that started in London but then spread across numerous cities and towns.
The intelligence for the operation came from the Serious and Organised Crime Agency (Soca), which is now the National Crime Agency. The intelligence, which proved accurate, said Kevin Hutchinson-Foster was storing guns for Duggan.
The coroner said the Met either had details to locate Hutchinson-Foster or could get them as he was on parole. “I am therefore concerned that there may have been the opportunity for better liaison between the MPS and Soca, and for more focus on intelligence about Mr Hutchinson-Foster, with a view to locating the guns prior to Mr Duggan collecting one,” Cutler said.
“The MPS and Soca did not devise a strategy which focused on Mr Hutchinson-Foster and the guns and which was capable of leading to them being seized before one was collected by Mark Duggan.”
He said no law enforcement witness realised deficiencies in the planning and added: “If lessons are not learned I believe that circumstances creating a risk of other deaths will occur, or will continue to exist, in the future.”
The coroner also raised concerns that although the aim of the operation in which Duggan was killed was to take guns off the streets, little attempt was made to seize weapons believed to be held by Hutchinson-Foster.
“The operation then came to a halt. No attempt was made to retrieve other guns from Hutchinson-Foster, who was only arrested months later,” Cutler said.
Distrust of police put off independent witnesses, he added and said the aftermath of the shooting should have been video-recorded. He also raised concern about the handling of key exhibits, such as the shoe box the gun had been in, and the moving of the minicab Duggan had been using when police forced it to stop.