A step-father carried the secret of a toddler's murder for almost 50 years before the truth was uncovered by a witness aged four at the time, a court heard.
David Dearlove, 71, always maintained that Paul Booth, 19 months, died as the result of an accidental fall from his bed on October 1 1968, when he had in fact swung the boy into a fireplace, it is alleged.
But the true story was revealed, jurors were told, when Paul's brother Peter - just four when he died - saw a faded photograph of the toddler being held by Dearlove on Facebook in 2015.
For years, a jury heard, Peter had been haunted by the memory of what he saw through a gap in a door on the night Paul died - Dearlove swinging his brother by the ankles and hitting his head against a fire surround.
Peter demanded Dearlove's family take down the photograph and then told his cousin what he had seen that night 47 years earlier. The cousin told the police that Paul's death had actually been a murder and an investigation was launched.
The death of Paul Booth had been no accident, it had been as the result of a deliberate act. It had been murder
Prosecutor Richard Wright QC
Dearlove, who split from Paul's mother Carol Booth in 1970, had long since left the Haverton Hill estate in Stockton-on-Tees.
He was arrested at his home in Great Yarmouth, Norfolk, where he lived with his wife of 36 years, and was charged with Paul's murder and accused of cruelty to his older siblings, Peter and Stephanie.
Prosecutor Richard Wright QC told Teesside Crown Court: "In the early evening of October 1 1968 a young mother knocked at the door of her neighbour on a long since demolished housing estate at Haverton Hill.
"The woman was called Carol Booth and she was knocking to ask her neighbour to come quickly to her house. The reason for the emergency was that one of her children, a toddler of about 18 months called Paul had stopped breathing.
"The neighbour rushed to the house. There was unconscious on the settee the young child, Paul Booth. He was obviously gravely unwell and his step-father, the defendant David Dearlove, appeared to be making an effort to resuscitate him.
"A doctor was called to the house and he in turn called an ambulance. The toddler Paul Booth was rushed to hospital where doctors did all they could to treat him during the night. Ultimately, the treatment available could not save Paul Booth and the little boy passed away later that evening.
"There was and is no doubt as to what medical condition caused the death of Paul Booth. He died because of a severe injury to his brain that had itself been caused by a fractured skull. The real issue then in 1968 as now nearly 50 years later in 2017 is what caused that injury?"
Despite a post mortem finding evidence of old bruising on Paul's body, and evidence from family members that the toddler had suffered injuries when left alone with Dearlove, he was not arrested. An inquest subsequently returned an open verdict and the case was closed until March 2015.
Mr Wright told the jury: "No police action was taken against Carol Booth or David Dearlove and the case was effectively closed.
"So it remained for nearly 50 years until March 30, 2015. It was on that day that a cousin of Paul Booth contacted the police. She was making the call on behalf of Paul's brother Peter, the little boy who had been almost four years old on the night his brother died.
"The police interviewed Peter as a witness for the every first time in 2015; he had not been asked to give any account in 1968. What he told in that interview is what has led us here, 50 years on, to the trial of David Dearlove for the murder of his stepson Paul Booth.
"Peter told the police that the death of Paul was not the result of an accidental fall out of bed. He had in fact seen how Paul came to be injured when he had crept downstairs to get a drink that October night.
"Through a gap in the door into the sitting room he had seen David Dearlove swinging Paul Booth around whilst holding onto his ankles and had watched as his step father smashed the little boy's head into the fire surround, causing the fatal injury to his skull by the impact.
"The death of Paul Booth had been no accident, it had been as the result of a deliberate act. It had been murder."
It was the modern medium of Facebook that led to Dearlove's arrest, the court was told.
Mr Wright said: "The catalyst for his complaint in 2015 appears to have been his seeing a photograph posted on Facebook of his dead brother being held by David Dearlove.
"Peter contacted David Dearlove Jr and demanded that he remove the photograph and then spoke with his cousin Tracy about why he was so upset, telling her that he had seen David Dearlove swinging Paul by his ankles in the front room, then striking his head off the fire surround causing the fatal injury.
"He had been extremely scared of David Dearlove because the violence in the household extended not just to David Dearlove hurting Paul but also to him regularly assaulting Peter."
Dearlove denies the murder and unlawful killing of Paul Booth and further denies three charges of child cruelty - defined as assault, ill treatment or neglect causing injury to health - in 1967 to 1968.
The trial, expected to last three weeks, continues.