The force said last night that an internal inquiry would be held into its handling of the case, looking specifically at procedures used in interviewing George Heron. But a spokesman said detectives were not looking for anyone else in connection with Nikki's death.
Her family is to appeal to the Lord Chancellor's Office to hold an inquiry into why the trial judge did not allow certain police interviews to be put before the jury.
Mr Heron, 24, who admitted he knew Nikki but denied in court that he had murdered her, was acquitted by a jury at Leeds Crown Court on Sunday. Mr Justice Mitchell refused to allow certain tape recordings of police interviews with Mr Heron to be put before the jury. Consequently, the jury did not hear his admission on one recording that he killed Nikki last November.
The judge ruled that the evidence was inadmissible and criticised the police for 'oppressive questioning' and of misrepresenting evidence, charges the force denies.
Nikki was stabbed 37 times and her body left in a deserted warehouse in Sunderland, near her home on the Wear Garth estate where Mr Heron also lived. After his arrest, members of his family had to move out of the area.
Yesterday, Mr Heron told the Sunderland Echo that he was now in hiding at a religious retreat in the north of England, and 'living in fear'. He said he had received death threats and added that he intended to sue the force because that was the only way to clear his name. He said he had only confessed after four days of 'interrogation', during which time his sister and her boyfriend were arrested for allegedly assisting an offender.
'It was just a one-sided battle which I wasn't going to win. They (the police) were oppressive and using members of my family. It was getting to me so I decided enough was enough. I didn't want my family to suffer. I knew it was me they wanted, so I gave them what they wanted. I was at the point where I wasn't sleeping properly. I wasn't eating properly and I wasn't thinking at all.'
Mr Heron said he wanted Nikki's family to know that he did not kill her. He hoped the real killer had the nerve to own up.
Detective Chief Superintendent Barry Stewart, the head of Northumbria CID, said yesterday that there was no question of his force 'looking for anyone else'. There would be a full review of the techniques used by his officers, but again he did not accept that these had been oppressive.
Nikki's mother, Sharon Prest, 27, said yesterday that as far as she was concerned, Northumbria Police had done their job perfectly well. 'I think it is completely wrong for him to announce that he is going to sue the police. How does he imagine that makes us feel?'
On Sunday night a firebomb, which failed to ignite, was thrown at the Sunderland offices of the firm of solicitors that represented Mr Heron. Police are patrolling the Wear Garth estate, and they have appealed to residents not to take the law into their own hands.