A police force has been accused of "individual and systemic failings" after a couple were murdered following an 18-month terror campaign.
Maureen died in the fire and her husband Alex died in hospital
Maureen and Alex Cochrane were killed in an arson attack on their Manchester home. Their daughter Lucy was hurt.
Greater Manchester Police (GMP) failed to visit them after a previous attack attempt, said the Independent Police Complaints Commission (IPCC) report.
GMP said it planned to examine the content of the report in detail.
Natalie Connor, 18, plotted with her parents to pour petrol through the letterbox of the Cochrane's house and set it alight - an act carried out by her father.
Michael Connor and his ex-wife wife Jane, both 40, were jailed for a minimum of 32 years each for the murders. Natalie was jailed for 11 years for manslaughter, grievous bodily harm and arson.
The Connors plotted to set the house alight
During their trial, the court heard the family had been subjected to a campaign of harassment from the Connors after the two daughters fell out.
The IPCC report criticises GMP for failing to record complaints from the Cochrane family and for not replying to a letter from them outlining the harassment.
It recommends one constable be given a formal written warning and an inspector "advised" over his conduct.
The report concludes it was "wrong" for an officer who had been dealing with the Cochranes not to visit their home after the first arson attack.
He was also "wrong" to give Mrs Cochrane his mobile number as it meant GMP's complaint "recording systems were circumvented".
The police watchdog said GMP's handling of the case was marked by "a failure to identify the Cochranes as victims" and "a general failure to interpret the incidents as harassment".
Lucy Cochrane survived the fire
Naseem Malik, IPCC Commissioner for the North West, said: "No-one can ever say that there would have been a different outcome had Greater Manchester Police acted differently.
"However it is clear from our investigation that there were failings in the duty of care this family received."
Although some of the actions were well intended, they exacerbated the failings in the recording systems, Mr Malik said.
"The Cochranes were victims of harassment but were not identified as such and therefore Greater Manchester Police failed to give them the service they needed," he added.
Ch Supt Alan Cooper, of Greater Manchester Police, said the officer involved believed he was going "above and beyond" what was expected by giving the family his phone number.
"We welcome the fact that the IPCC clearly recognises in the report that there is no way anyone could have predicted that what started as a minor school incident could have ended in such tragedy, " said Mr Cooper.
"They also acknowledge that those solely responsible for their deaths were Michael, Jane and Natalie Connor.
"We will, of course, carefully examine the contents of the report in relation to our crime recording systems. However we would stress they are subject to regular internal inspection.
"At all times, GMP officers acted in good faith with the Cochrane family, and it is a matter of profound sadness Maureen and Alex lost their lives in this most distressing of ways," he said.