When Ian Tomlinson's widow watched video footage of his last moments alive for the first time on a laptop 16 months ago, she was speechless.

Julia Tomlinson had been told by police her husband had died of natural causes as he tried to get home through the G20 protest in London, and there was nothing suspicious about the death.

But as she watched footage shot by a New York fund manager and handed to the Guardian, which was conducting its own inquiry, a different story unfolded.

Tomlinson, hands in pockets, was walking away from police. Julia Tomlinson winced as, repeatedly, she watched as an officer who was not displaying his badge number, and whose face was concealed behind a balaclava, lunged at her husband from behind and, without provocation, struck him on the leg and pushed him to the ground.

Eventually, through tears, Julia Tomlinson said she wanted justice. Yesterday, she learned there would be none.

At a meeting with the Crown Prosecution Service (CPS), she and her nine children were told of two seemingly incompatible conclusions: first, prosecutors believed there was sufficient evidence to prosecute the officer; but second, owing to both the time limit for common assault charges and medical uncertainties – crucially, the contested findings of a forensic pathologist who is now the subject of an inquiry by the General Medical Council – prosecutors did not believe there was a prospect a jury would convict the officer on any charge.

The merits of the CPS arguments put forward for not prosecuting the officer – a van driver from the Metropolitan police's territorial support group (TSG) – had by tonight already become the subject of intense controversy. For the victims though, it was not a huge surprise.

A close-knit working-class family from the Isle of Dogs, east London, Tomlinson's relatives long believed they would be deprived of justice by a system of police accountability they suspected was biased in favour of those who carry the batons.

The police disregard for Tomlinson was evidence on footage of the aftermath of the attack, which left him lying on the ground in front of a line of riot police shortly after 7.25pm on 1 April.

None of the officers went to the aid of the 47-year-old, who was clearly in distress. Instead, it was left to a bystander, Alan Edwards, 34, to lift him to his feet.

Edwards would later recall his conversation with Tomlinson. "I said: 'You OK, mate?' He said: 'No, I live down there – that's where I live. I can't get there any other way. I'm trying to get home.'"

Looking disoriented, Tomlinson then stumbled 100 yards down the road before collapsing and dying.

The initial police response was to accuse protesters of wrongdoing. Within four hours, Scotland Yard had released a statement saying officers had gone to the victim's aid and called an ambulance, and were attempting to save his life with cardiopulmonary resuscitation when they were impeded by protesters who attacked them with "a number of missiles – believed to be bottles".

In fact, it is no longer thought Tomlinson's treatment was impeded when two, probably plastic, bottles landed nearby. Protesters placed Tomlinson in a recovery position and called the ambulance before police arrived.

In the following days, City of London police, which was investigating the death, would receive information from witnesses that suggested Tomlinson might have been assaulted by an officer. His family were not told about this, and were advised instead that he had died after being caught up in a fracas prompted by anarchist demonstrators attacking police. Police told them witnesses had seen the newspaper seller simply "run out of batteries".

Two days after the death, in what would become a crucial decision, the City of London coroner, Paul Matthews, requested that forensic pathologist Freddy Patel conduct a postmortem.

Patel had once been reprimanded by the General Medical Council over his conduct, and is not thought to have had a police contract at the time of the Tomlinson case. The Met had written to the Home Office four years earlier raising concerns about Patel's work in a number of cases.

Leading forensic pathologists say privately that they were astounded to learn such a controversial postmortem would be entrusted to an expert who was no longer thought to be actively dealing with suspicious cases.

Patel has since been barred from the Home Office register of accredited forensic pathologists and from carrying out postmortems in "suspicious death" cases.

Matthews has declined to say why he chose Patel. One theory was that the coroner was recommended Patel's services by City of London police. The force has declined to comment.

It was Patel's findings that enabled police to assure the family, who were prevented from viewing the body for six days, that there was nothing contentious about the death.

Telling the family Tomlinson had "died of a heart attack", police made no mention of significant injuries found on his body, including blood in his stomach, bruising, and a dog bite on his leg.

Police also briefed journalists, erroneously, that there was nothing suspicious about the death and that the victim's family were "not surprised" he had died because of prior medical conditions.

Tomlinson's family deny ever having said this, and for their part claimed they were "badly misled" by police. They believe they were the victims of a cover-up and that some police simply refused to countenance that an officer was involved in the incident. When a senior City of London investigator watched footage of the attack, he said Tomlinson's assailant could be a member of the public "dressed in police uniform".

However, it is the Independent Police Complaints Commission (IPCC), the watchdog with the job of investigating police, that may ultimately be taken to task over failings in the case.

The first, crucial days of the inquiry were left to City of London police, in whose jurisdiction the death took place.

Three days after the death, after receiving word of Patel's postmortem, the IPCC drafted a report into the death, concluding there was "no evidence" of police involvement in the death.

Despite evidence of contact with police, the IPCC resisted calls to launch an inquiry for six days. It concluded that Tomlinson died of a heart attack after being caught up amid charging protesters.

The draft IPCC report was read over the phone to Tomlinson's family, but was then shelved at the last minute.

The IPCC only agreed to investigate the case when the Guardian released footage of the incident and handed investigators evidence that contradicted the police version of events.

The footage instantly made headlines around the world, and would eventually transform perceptions of British policing, leading to two parliamentary inquiries into the Met's handling of the protest and a national review of policing.

The officer caught striking Tomlinson came forward and was questioned under suspicion of manslaughter.

In August last year, the IPCC announced it had completed its inquiry and handed a file to the CPS. The family were told in a private meeting with IPCC officials that, while they were not responsible for deciding on a prosecution, investigators believed there was sufficient evidence to charge the officer with manslaughter.

When two IPCC investigators flew to New York to collect the original video footage, the investment fund manager was surprised that they arrived without basic equipment such as a tape recorder.

Despite promises of a swift decision, concern over Patel's evidence led to months of delays.

Almost 300 complaints were lodged about police behaviour during last year's G20 protests, arguably the most controversial large-scale policing operation in the last decade. Yet after dozens of inquiries, one trial and one of the largest inquiries in the history of the IPCC, not a single officer has yet faced serious disciplinary action. Two officers have received written warnings.

The fund manager who shot the Tomlinson footage – a man who, like Tomlinson, had no affinity with the anti-capitalist ideology of many demonstrators that day – expressed disappointment tonight. He said: "With this decision, the green light has been given. Future aggressive police behaviour towards ordinary citizens will be tolerated, overlooked, and excused."