A report which concluded that senior detectives investigating the death of 13-month-old Poppi Worthington were "unstructured and disorganised" has left her mother "profoundly disappointed and distressed", say her lawyers.
The Independent Police Complaints Commission (IPCC) also indicated there was enough suspicion "on day one" to arrest her father, Paul Worthington, over the death of the toddler who collapsed suddenly at the family home in Barrow-in-Furness, Cumbria, in December 2012.
A family court judge previously ruled that Mr Worthington probably sexually assaulted her before her death. However, Mr Worthington strenuously denies any wrongdoing and the Crown Prosecution Service has said there is "insufficient evidence" to charge him.
The 2014 civil fact-finding judgment also concluded that Cumbria Police conducted no real investigation for nine months after the senior investigating officer considered a pathologist may have "jumped to conclusions" when suggesting an unlawful act may have taken place.
Those findings were passed to the police watchdog which finally published its conclusions on Friday - two years after reaching them.
In response to the report, Fiona McGhie, an expert civil liberties lawyer at Irwin Mitchell, speaking on behalf of Poppi's mother - who cannot be named for legal reasons - said: "Poppi's mother is understandably deeply concerned by the findings of the IPCC.
"She has anxiously awaited answers as to what happened to Poppi on that fateful day.
"To learn that the actions of senior investigators within Cumbria Constabulary may have contributed to the agonising delays she has endured has left her deeply and profoundly disappointed and distressed.
"She hopes that the long-delayed inquest, now scheduled for May this year, will give her the answers she deserves and allow her to access justice so Poppi can be at peace."
The IPCC said police did not adequately investigate whether Poppi had been abused despite concerns raised by a hospital doctor, the Home Office pathologist Dr Alison Armour days later at the post-mortem examination and in a later conversation between Dr Armour and the then lead officer Detective Inspector Amanda Sadler.
Lead investigator for the IPCC, Tim Kimber, said the post-mortem - which eventually concluded the cause of death as "unascertained" - was the pivotal point of the inquiry as Ms Sadler considered Dr Armour's opinion and a second doctor's suggestion that Poppi's injuries may be consistent with constipation.
He said it was arguable that Ms Sadler not seeking clarification while she was in the room with the two pathologists led to "unclear lines of inquiry from that point".
The investigation was passed to Det Supt Mike Forrester who, the IPCC said, appeared to jump to conclusions and disregard what was at least a feasible line of inquiry.
The report stated: "The IPCC investigator considers that there is substantial evidence upon which it could be concluded that these two lines of inquiry were not pursued equally and appropriately and that there was more focus on establishing a natural cause of death.
"Overall the IPCC investigator considers that there is substantial evidence available to support the contention that the reason this case has still not reached a resolution more than two years on from the death of Poppi is because of the unstructured and disorganised approach taken by D/Supt Forrester and DI Sadler when investigating her death, coupled with the argument that prior to the pathologist report being received they did not conduct a criminal investigation despite there being significant suspicious circumstances from the outset."
Although the IPCC accepted a six-month delay in providing the full post-mortem report presented a difficulty to the investigating team, it concluded there was enough evidence to have justified an arrest either on the day of Poppi's death or shortly after the post-mortem.
Mr Kimber said there were concerns that Poppi's father was the last person taking care of her before her death and a decision had been made to obtain a penile swab from him.
He stated: "Not only were there suspicious circumstances, there was also a suspect on day one.
"Despite the level of suspicion, and there being a suspect, a crime was not recorded until 28 August 2013, which is the day the parents were arrested."
He said that protecting the siblings of Poppi would have satisfied the necessity grounds for arrest.
The IPCC also identified that lower ranking detectives felt uncomfortable with how the investigation progressed and they thought they were kept out of the loop.
The probe was not run on any case management system and "key decisions and rationale for them were not documented clearly and, as such, the audit trail for the investigation is unclear".
The IPCC also found evidence that the family home on the morning of the death was not adequately preserved and searched, resulting in a nappy that Poppi had been wearing being lost as potential crucial evidence.
Mr Kimber concluded that both Mr Forrester and Ms Sadler had cases to answer for gross misconduct.
Mr Forrester retired so no further action could be taken.
Ms Sadler was subjected to a disciplinary hearing last year where gross incompetency was proven and she was demoted in rank. She too has since retired from the force.
IPCC commissioner Carl Gumsley said: "This is an extremely sad case.
"It is clear from the evidence presented that Cumbria Constabulary's original inquiry into Poppi's death was not fit for purpose with many inquiries not being actioned for several months.
"The entire inquiry did not reach a resolution until almost two years later.
"These findings posed serious questions for Cumbria Constabulary.
"I have been assured that the force has since attempted to address these failings. It is absolutely crucial that the force continues to improve to ensure this never happens again.
"I appreciate there is huge public interest in this case and there has been keenness for comprehensive details about the circumstances surrounding Poppi's tragic death to be made available.
"The IPCC investigation concluded in March 2015, however, detailed findings could not be published until now as we had to ensure we did not prejudice the ongoing criminal, disciplinary and inquest processes.
"I know this has caused some frustration. However, it was important that these judicial processes, designed to seek out the truth, were properly protected."
Poppi's death had been shrouded in secrecy with the civil court judgment by Mr Justice Peter Jackson kept private so as not to prejudice any criminal proceedings, and the first inquest into her death lasting just seven minutes.
A second inquest is due to take place in Kendal in May