Image caption As well as being a journalist, Jill Dando presented series such as travel programme Holiday

The detective who led the inquiry into Jill Dando's murder has told the BBC her case will never be solved.

Hamish Campbell told a documentary to mark 20 years since the newsreader's shooting: "Do I think somebody will come back to court? Probably not, no."

His team arrested Barry George in 2000, one year after Dando, 37, was killed on her doorstep in Fulham, west London.

Mr George was convicted of murder and spent eight years in jail, before being acquitted at a retrial and released.

In the second trial, the jury accepted that one particle of gunshot residue found in the pocket of a coat police retrieved from Mr George's house was insufficient to place him at the scene of the murder.

It had been the key forensic evidence relied on by detectives.

Ms Dando was a popular presenter of programmes such as the Six O'Clock News, Breakfast News, Crimewatch and Holiday.

The BBC has said "previously unseen archive and photographs" will feature in the documentary, alongside interviews with Jill's colleagues, friends and family members.

The film reveals the decision-making behind the scenes of the murder inquiry.

Mr Campbell told the programme that senior officers were pressing him on the likelihood of the case being resolved.

"Sometimes I felt we were a day away from solving it and other times, I thought 'no, we're a long way away'," he said.

Media playback is unsupported on your device Media caption Hamish Campbell: "Do I think somebody will come back to court? Probably not, no."

More than 2,000 people had been named as potential suspects.

"Some actions to trace and eliminate one person might take a day. One action might take two weeks," Mr Campbell said.

"But there's thousands of them and that's the issue of managing stranger homicides."

The film will also show how BBC director general Tony Hall, then the corporation's head of news, was targeted with three telephone threats in the weeks after Ms Dando's murder.

"I listened to the voice of one of them, which said basically, I was next," he told the programme.

Lord Hall said he had "no idea" if the threat was real.

"There are often copycat things that happen after these sorts of events, and the police took it seriously."

The Murder of Jill Dando will be shown on BBC One at 21:00 BST on Tuesday 2 April.