A new BBC One documentary titled The Murder of Jill Dando airs tonight at 9pm, marking the 20th anniversary of the TV presenter’s death.

Although it offers no fresh leads, it promises to tell “the full story” behind the killing of the Crimewatch host using previously unseen archive and interviews with Dando’s colleagues, friends and close family.

The 37-year-old was killed by a single bullet fired at her head as she reached the front door of her house, on 26 April 1999.

“The murder, in the quiet, leafy street where the cherry trees were in blossom yesterday, remains one of the most high-profile unsolved murders,” says The Times.

Barry George, a convicted sex offender, was found guilty of her murder in July 2001, but was acquitted six years later after gun residue found on him during the forensic investigation was discredited.

The case is still open, but the detective who led the inquiry into her murder told the BBC recently the case will never be solved.

The events of 26 April 1999 have been repeatedly poured over for any clues that could help police in their investigations.

Here is what we know:

At 10.03am a postman delivering mail to Dando’s house in Fulham noticed he was being watched by a dark-haired man in a suit. At the same time, a traffic warden patrolling the area spotted a blue Range Rover parked illegally. She began to note down the licence number but stopped when the driver protested.

Both the man in the suit and the Range Rover were seen by several different people over the next hour.

At 11.30am Dando walked to her front door and was shot once in the head. Two witnesses reported seeing a man with thick black hair and a waxed jacket running away from the house.

Minutes later, CCTV images showed a blue Range Rover travelling at high speed down Fulham Palace Road away from the scene of the crime.

At 11.40am a man was spotted crouching by some railings in a nearby park, furtively talking on a mobile phone, and ten minutes later a motorist was forced to brake hard when a man ran across the road away from Dando’s street.

At 1.03pm Jill Dando was declared dead at Charing Cross Hospital.

Here are six theories about Dando’s death.

Gangland hit

The nature of the shooting “appeared to have all the hallmarks of a professional gangland hit”, says the Daily Mirror. “An intelligence report named two men from one of London's most prominent crime families.”

In a 2017 ITV programme, an anonymous hitman told Mark Williams-Thomas, an ex-Surrey Police officer who is now an investigative journalist, that he knows the criminal behind the killing but wouldn’t name him for fear of recriminations.

The hitman was shown a list of 100 names that Williams-Thomas had been given by police investigating Dando’s murder.

“There are names here that I recognise, and there's one in particular that stands out to me,” the hitman said. “But I wouldn't identify that person because it's very dangerous. I'm sure that they would come after me.”

In an interview on This Morning, Williams-Thomas added: “I am very confident the killer or the person who organised the killing is in that inquiry team database.”

Serbian hitman

In 2012, the widow of Slavko Curuvija, a prominent Serbian journalist, said that Dando became a target for Serbian warlords after presenting a BBC appeal on behalf of Kosovan-Albanian refugees driven from their homes by militias backing Yugoslav President Slobodan Milosevic.

Branka Prpa was with Curuvija when he was shot dead outside their home in Belgrade just 15 days before Dando was killed. According to The Daily Telegraph Prpa said: “I think there is a link between Dando and Curuvija. I think they were both executed.”

Detectives investigating Dando’s murder also received a tip-off saying Serbian criminals based in the UK carried out the hit under orders from the Serbian regime.

The Mail on Sunday says “the ‘Serbian connection’ became a cornerstone of the defence case in the 2001 Old Bailey trial of Barry George”. His barrister Michael Mansfield told the court that the National Criminal Intelligence Service had a dossier suggesting her killing was in retaliation for the Nato bombing of the Radio Television of Serbia headquarters three days earlier.

Mansfield said the notorious Serb warlord and underworld boss Zeljko Arnautovic, known as Arkan, had put out a contract on the then-BBC director general Sir John Birt. “His security had been stepped up as a result - so the target was switched to the unguarded Jill Dando,” says the paper.

Arkan was shot and killed nine months after Dando died, while the only other Serb believed to have been named in the 2014 police cold case review insists he was not hired to kill the presenter.

Speaking under the pseudonym Milan Mitrovic, he told the Mail on Sunday: “I had been implicated in other things in my life but never in the murder of an innocent woman. It wasn’t nice being accused of such a heinous crime.”

He confirmed that he had been questioned by Scotland Yard detectives years ago, but said the stamps in his passport proved he was in Macedonia on the date of Dando’s death.

Others have also disputed the Serbian theory. BBC World Affairs editor John Simpson, who was in Belgrade in April 1999 covering the Nato bombing campaign, claims it would have been much easier to target him if Arkan or anyone in the regime had wanted to kill a prominent BBC journalist.

In an analysis of the evidence presented to police in May 1999, Dr Adrian West, one of Britain’s foremost forensic criminal psychologists, also questioned why, if Dando’s murder was politically motivated, there had been no credible public claim of responsibility.

IRA

Another theory is that senior paramilitaries from the IRA chose Dando as a target “because of her links to police through her work presenting Crimewatch”, says the Mirror.

A letter was found admitting to the killing among the 52,000 documents that were made available to Barry George’s legal team during his second trial.

In it Wayne Aird, who was serving life for killing a man two months after Dando’s death, confessed to being part of an “establishment cover-up” and said that the IRA was being allowed to get away with the murder to avoid harming the Northern Ireland peace process.

Paedophile ring

In 2014, a former colleague of Dando’s came forward and said she was trying to expose a VIP paedophile ring just months before her death.

According to the Daily Express, the source said Dando raised concerns to her BBC bosses about allegations of sexual abuse happening at the corporation.

The source said: “I don't recall the names of all the stars now and don't want to implicate anyone, but Jill said they were surprisingly big names.”

But the BBC has said that it had seen nothing to substantiate these claims.

Obsessed stalker

The theory first adopted by police was that of a crazed opportunistic individual.

The only certain sighting of the killer - a 6ft (183cm) white man aged around 40 - came from Richard Hughes, Dando’s neighbour, who said he heard a surprised cry from the presenter, “like someone greeting a friend”, but heard no gunshot.

This statement has led some to suggest that Dando knew her killer. The Guardian reports that “she had been irritated by a number of obsessive fans”.

‘Joe’ the Spanish barman

A report from the now defunct National Criminal Intelligence Service suggested Dando’s murder could be traced back to a gunman called Joe who worked in a bar in Spain and had links to murderer Kenneth Noye.

Noye was sentenced to life in prison for a 1996 road rage killing with the help of a Crimewatch appeal.

The report said: “Joe runs a bar in Tenerife, frequented by leading ex-pat criminals. He's described as a frustrated gangster reputed to owe money to Kenny Noye. There’s been talk Joe has been keen to rehabilitate his reputation with gangster creditors.”

The Daily Mirror says that Joe “allegedly came to the UK specifically to carry out the crime”, although it “appears that Joe was never traced”.