A man has been arrested over a missing laptop used by Poppi Worthington's father to watch pornography on the night the 13-month-old was sexually abused and died.
Paul Worthington, 50, told police he viewed "adult X-rated" porn on the laptop but officers did not seize the computer and it has never been recovered.
It was one of a series of failures by Cumbria Police who lost vital evidence during their botched investigation, which meant no-one has been prosecuted over Poppi's sexual abuse and death.
Cumbria Police made no "pro-active" attempts to trace the laptop, which Mr Worthington said he had sold to another man. But after criticism of the investigation, the force made an arrest, last week, more than five years after Poppi's death.
A spokesman said: "Officers from Cumbria Police arrested a 39-year-old man, from Barrow-in-Furness, on suspicion of assisting an offender and possess with intent to supply a controlled drug of Class A - MDMA.
"The male is co-operating with enquires and has been released under investigation. The arrest, made on Thursday 15th March, relates to a statement previously made to officers investigating the death of Poppi Worthington concerning the location of a laptop."
Mr Worthington will not face any legal action, the Crown Prosecution Service (CPS) ruled last week, despite a court three times having found he probably sexually abused his daughter before her sudden death.
He denies any wrongdoing and is now living in hiding with the help of police protection. His "fit and active" daughter suffered injuries to her bottom and woke up screaming at about 5.30am on December 12 2012 at the family home in Barrow-in-Furness.
Earlier he had been watching porn on the laptop in bed before taking Poppi out of her cot and putting her in his bed. Paramedics took the apparently lifeless girl to Furness General Hospital at 6.11am but she never regained consciousness and was pronounced dead shortly after 7am.
In January the senior coroner for Cumbria David Roberts concluded at the end of the three-week inquest into her death that Poppi was anally penetrated in her father's double bed at the family home, though her death did not result from the abuse.
Mr Worthington refused to answer questions about Poppi's death 252 times at her inquest, so as not to potentially incriminate himself. The Crown Prosecution Service said last week that Mr Worthington would not face any legal action and there would be no fourth review of the case.