A woman has been jailed for the murder of a 19-year-old man in Sunderland 14 years ago, a crime that remained unsolved until she walked into a police station and confessed.

Scott Pritchard was found with serious head injuries by the back door of his home in Hendon on 7 January 2004, and later died in hospital.

Despite a police investigation that involved 300 officers, 1,600 statements and 4,000 exhibits, the case remained open until Karen Tunmore, 36, a children’s football coach, admitted to the murder in July.

On Monday at Newcastle crown court she was given a life sentence with a minimum term of 17 and a half years.

Karen Tunmore. Photograph: PA

Tunmore attended Middle Engine Lane police station in Wallsend at 10pm on 31 July and said she had murdered Pritchard and could no longer stand the guilt.

“To get that call so long after the event, you have that feeling of it being a bit incredible,” DCI John Bent, of Northumbria police, told the Guardian. “But actually once we started taking details from her she had a chilling amount of accurate information that was never released to the media.”

Tunmore was charged and pleaded guilty at a bail hearing at Newcastle magistrates court on 8 August.

She said that on the day of the murder she was with a man she called Steve to whom Pritchard owed money. The pair asked Pritchard for cash and when he said he didn’t have any she “saw red” and hit him three or four times over the head with a baseball bat, a story consistent with the pathologist’s report.

Pritchard was using crutches at the time because of a broken foot, and Tunmore told police he was sitting down because he was tired when she attacked him, a detail consistent with the pattern of blood spatter. She said she later threw the bat in the Tyne at North Shields fish quay and sold her car, which had blood stains.

In 2005 Pritchard’s father, Robert Stacey, was charged with his son’s murder, but the case was dropped when the Crown Prosecution Service decided there was not enough evidence against him. Bent described Stacey’s arrest as a “sad twist in the tale”.

“He was arrested on circumstantial evidence alone,” he said. “There was no direct forensic evidence or witness testimony that put him at the scene. There was the suggestion that the pair may have fallen out, which could have been a motive.”

Stacey, now 64, is currently in hospital after suffering a heart attack. In a victim impact statement read to the court, Stacey said being charged with his son’s murder had stunned him. He spent 16 months in Durham prison before the charges were dropped.

Stacey described being unable to return to Hendon because of abuse he received from people who thought he was guilty of murder. He said a rock was thrown through his window and he received threats that his house would be burned down.

Police said Tunmore was very remorseful about what had happened and had apologised for the grief she had put the family through. “Living with a horrific murder like this has weighed heavy on Tunmore’s conscience,” said Bent.

He said police were looking to speak to Steve, the man Tunmore was with that day. She said she had met him through attending car rallies and he was a member of the Mad Dogs criminal gang that operated in Hendon at the time of the murder.

He was 19 or in his early 20s at the time and had dark hair and a distinctive scar on the left side of his head above his ear.

“We think she knows who this person is but she won’t tell us because she says she’s not going to be a grass. Steve might not be his name,” said Bent. “She says that after they got rid of the car they never spoke again.”

The court heard that Pritchard’s mother, Kathleen Pritchard, had always thought his father was responsible for his death and she was in a state of shock after Tunmore admitted to the crime.

She paid tribute to “a wonderful son who had his best years still ahead of him”.

“He had grown into a kind, confident and well-liked young man who was cruelly taken from us 14 years ago,” she said. “No sentence today could have brought Scott back, and the pain we have felt every day since he was murdered will never subside.

“All we’ve ever wanted since his death was justice. While no sentence could take away the hurt that we have felt since Scott was murdered, we can hopefully look forward knowing the individual responsible can never destroy another family.”