David Boyd, 55, was found guilty of murdering seven-year-old Nikki Allan in May last year - 31 years after the schoolgirl's body was found in a derelict basement in Sunderland's East End in 1992.

The killer was sentenced at Newcastle Crown Court to life in prison with a minimum term of 29 years.

In the initial investigation, Boyd, the boyfriend of Nikki's babysitter and who lived in the same block of flats, was not treated as a suspect by police, even after he was convicted of a child sex offence in the late 1990s.

The police probe instead focused on an innocent man, George Heron.

Heron admitted to Nikki's killing during a Northumbria Police interview after three days of questioning, but was acquitted after a judge at his murder trial in 1993 ruled police tactics were "oppressive".

It was only a DNA breakthrough in 2017 that led to Boyd's eventual conviction.

Nikki's mother, Sharon Henderson, never gave up her fight for justice for her daughter, making her own investigations and repeatedly calling on the force to reopen the inquiry.

She received an apology from Northumbria Police following Boyd's conviction, while the force also apologised to Mr Heron, who had to leave Sunderland despite being cleared.

The case is now the subject of an investigation into two complaints to the police watchdog - the Independent Office for Police Conduct (IOPC).

It will look at the actions and decision-making of police involved in the murder investigation and a subsequent homicide review.

This will include the identification and elimination of suspects during the original investigation, as well as whether there were missed opportunities to identify Boyd using DNA analysis sooner, the watchdog said.

Ms Henderson said she hoped the investigation would "lift the lid and expose incompetence and worse".

"This is the next stage of my campaign to get truth and justice and to hold those responsible accountable," she said.

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The IOPC was asked to investigate by the Centre for Women's Justice (CWJ) on behalf of Ms Henderson.

Her solicitor Harriet Wistrich, who is director for CWJ, said: "It is extremely concerning that a child murderer who lived only three doors away from Nikki's home, failed to come under the spotlight of the police for nearly three decades.

"Instead the police insisted their prime suspect had been acquitted on a technicality and failed to explore other suspects.

"It is outrageous that the bereaved mother of the deceased child should have suffered any blame or criticism and had to take increasingly desperate measures to get the police to do their job.

"I hope the IOPC will leave no stone unturned in exposing incompetence and wrongdoing."

IOPC regional director Emily Barry said: "My sincere sympathies are with Nikki's family, who have lost a loved one in such horrific circumstances and then endured a 30-year wait to see justice served.

"Our investigation, which will be carried out independently of police, will be thorough and ensure the actions of police are thoroughly scrutinised."