Hamish Campbell thinks it is unlikely now that any new suspects will be taken to court.
Ms Dando, 37, was shot on the doorstep of her house in Fulham, west London, in April 1999.
Barry George was arrested in 2000 and served eight years in prison for her murder before being acquitted and released after a retrial.
In a documentary marking 20 years since Ms Dando’s death, Mr Campbell said he doubts the case will be solved.
He told the BBC: “Do I think somebody will come back to court? Probably not, no.
Mr Campbell continued: “Sometimes I felt we were a day away from solving it and other times, I thought ‘no, we’re a long way away’.
“Senior officers were asking ‘what are the likelihoods of this case being resolved?’
“We had over 2,000 people named as potential suspects or responsible. Some actions to trace and eliminate one person might take a day. One action might take two weeks.
“But there’s thousands of them and that’s the issue of managing stranger homicides.”
The documentary goes through the police inquiry and why they made the conclusions they did.
It shows how a particle of gunshot residue in the pocket of a coat at Mr George’s house was a key piece of forensic evidence.
At Mr George’s retrial it was accepted by the jury that one particle of gunshot residue was insufficient to place him at the scene of the murder.
The documentary also reveals how BBC director general Tony Hall, who was at the time Head of News, received threatening calls following Ms Dando’s murder.
He said: “We had three calls, as I recall, to the BBC switchboards in London and Belfast.
“I listened to the voice of one of them, which said Basically, I was next.
“I mean they were threatening me.
“I have no idea what that amounted to, Was it a real threat? Was it not a threat?”