The 37-year-old former Crimewatch presenter was shot dead on her doorstep in Fulham, London in April 1999 in a notorious case that has puzzled detectives for two decades.
Dando parked her car outside her home just after 11.30 after just doing some shopping.
As she approached her door a man came up behind her, quickly restrained her before shooting her at close range with a single bullet to the head.
The whole incident took 30 seconds from the moment she left her car, her attacker vanished quickly from the scene and the gun was never found.
Numerous theories including hitmen, criminal gangs and even a stalker are amongst those which continue to captivate and confound the nation.
In 2001, Barry George, a local man with learning difficulties, was convicted for her murder at the Old Bailey, but he was released in 2009 after an appeal found there was a lack of evidence supporting the Met Police’s case.
The documentary, which airs tonight at 9pm on ITV, also reveals new information found in boxes of evidence from the first trial, as well as an interview with Sir Cliff Richard, one of Jill Dando’s best friends.
However, one of the documentary's most riveting strands is a trail of evidence it follows to Belgrade in Serbia, looking at a theory which connects her murder to the Yugoslav Wars in the late 90s.
The documnetary details how at the time of Ms Dando's murder, Britain was one of the main partners in NATO's bombing campaign against Serbia, which was engaged in a campaign of ethnic cleansing against Kosovar Albanians.
Just three days before her murder, NATO bombs fell on the Serbian state broadcaster, RTS, killing 16 innocent people.
Three weeks before she was murdered, Ms Dando presented a charity appeal to raise money for Kosovan refugees.
The documentary uncovers what appears to be a potentially significant flaw in the investigation into her death Operation Oxborough.
Met Police said the case remains open so they can not comment other than to say detectives will explore any new information that becomes available.
The Serbian population are still to this day outraged by the attack, a local man telling reporter Julie Etchingham: “It would be the same if Moscow hit the BBC with a nuclear missile.
According to the documentary, the police files contain intelligence reports stating Ms Dando was murdered in revenge for the bombing but the Met Police investigation did not think the Serbia theory was credible.
The National Criminal Service, known then as Britain’s FBI, even released an intelligence report to investigators which alleged that Ms Dando was murdered in revenge of the RTS bombing.
Compelling evidence shows that a man appeared to have made a series of calls to the BBC, citing the murder as revenge for the bombing of the state radio and television service.
The documentary states that the police had details of these calls but that they never connected all of them to the same man.
By 3pm on the day of the murder, the man apparently made his first call. According to a summary of message found by the programme, he said: “RE THE MURDER OF JOURNALIST TELL YOUR PRIME MINISTER IN BELGRADE 15 KILLED SO 14 MORE TO GO.”
Just after 11am the day after the murder, the same man is believed to have made another call to the BBC, saying: "Yesterday I called to add a few more numbers to the list because your government and in particular your Prime Minister Blair murdered, butchered 17 innocent young people.
“He butchered, we butcher back. The first one you had yesterday. The next one will be Tony Hall.”
This call was taken sufficiently seriously by police to require increased security around Tony Hall, who was then the BBC’s head of news.
The police suspected the same man made another call with a similar message on April 28, saying: “Listen you and the BBC are the voice of your government that's why your reporter is dead because your government killed 17 innocent people.”
While the police were aware of the second two calls, it seems they did not connect them to the call on the day of the murder.
Phillip Ingram, a former colonel in British army intelligence, said the three messages, taken together, could have been a credible claim for the murder.
He says: “That’s significant because we would have expected someone to try and claim responsibility on the day of the murder and soon afterwards.
“If something that significant because it happened on the day of the murder has been missed then we have to ask the question what else could have been missed.”
The programme uncovers further potential links to Serbian warlord Arkan, as well as to major UK criminals including evidence about the bullet used in Ms Dando’s murder.
Jill Dando – The 20 Year Mystery will air on ITV at 9pm tonight (Thurs)