It's a brutal crime that's haunted the streets of Merseyside for decades. 

In 1986, Diane Sindall, 21, was raped and murdered by a killer dubbed the "Beast of Birkenhead".

The florist had just finished working a late shift at a pub. It was a part-time job that she had only taken on to make some money ahead of her upcoming wedding.

But on her way home, Diane's blue Fiat minivan ran out of petrol. She got out and started to walk.

Then, she was raped and savagely beaten to death. Her body was found in an alleyway the next day.

For decades, investigators from Merseyside Police thought they had caught her killer: a local man called Peter Sullivan, then aged 30.

Mr Sullivan had always maintained his innocence.

But last year, Mr Sullivan, now 68, was released after new tests showed his DNA was not present in semen samples collected at the scene.

By the point that his conviction was overturned, Mr Sullivan had spent 38 years in prison.

It is the longest miscarriage of justice in UK history.

Why there's renewed hope of a breakthrough

Now, Merseyside Police are searching once more for the person who killed Diane and working with Crimestoppers to offer a £20,000 reward for any information that would lead to an arrest and conviction.

"Back in 1986, a sample was recovered from Diane at the crime scene," explains Detective Superintendent Rachel Wilson from Merseyside Police.

"Now we've got the benefit of the enhancements in the DNA work, and we've identified a profile of a man from that sample."

DSI Wilson says this has become the "key line of inquiry" for the force.

"We need to identify who this DNA profile belongs to," she says.

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Of course, after nearly 40 years, police admit Diane's killer may be dead.

But officers say they remain "optimistic" they can identify her killer and bring justice for Diane's family after so many years.