His colleague, Detective Sergeant Steve Salton, said police exhausted investigations on several charges, including manslaughter, neglect and failing to provide care but their legal arm advised none of the charges stood a strong chance of being upheld in court.
"We can't prove the ingredients ... to get a conviction. We really wanted to charge, but we just don't have a charge we can do him with."
The file has been passed to the Ministry of Business, Innovation and Employment's health and safety section, which is investigating whether charges can be laid under the Health and Safety in Employment Act.
This week, Rapihana's mother, Audrey, said she was stunned with the decision.
"How many more have to die before something is done? They [Rapihana and Siddiqui] were adults as far as the system is concerned but I still had legal guardianship of my son," she said.
Siddiqui's mother, Tahira, was also unhappy no charges would be laid. Her son still had a scar on his hand and it took several months for him to recover from the emotional trauma of the blaze.
She did not know the caregiver had two autistic men in his care and would not have employed him if she had known. "I'm not saying he should be hanged but there should be some charges."
Her son survived only because the homeowner, who lived above the flat, rescued him by running back into the burning building to find a key for the garage door, which led to an internal door to the flat.
"Ali was screaming for help and suddenly she remembered there was a door from the garage into the house and she had a key for the garage. That's what she told me."
