Maria Voisin, the senior coroner for Avon, said there was a “catalogue of failures” that allowed Jason Conroy to strangle Melissa Mathieson at a care home in Bristol in October 2014.

The teenager was attacked by Conroy, then aged 18, at Alexandra House, which provided residential care for adults with autism and Asperger’s syndrome.

Conroy, now aged 28, was jailed for life for the sexually motivated killing.

Avon Coroner’s Court heard how he once tried to strangle a teacher so he could abuse her and had also attempted to kill his mother.

Conroy, originally from Guernsey, moved to Bristol from a boarding school in the Midlands following the incident with the teacher.

The school had commissioned a report from a forensic psychologist which concluded that he posed a physical risk to others because of his sexualised behaviour.

They warned the managers of Alexandra House about the risk, but this was not reflected in his care plans, which only listed basic information.

The directors of the company which ran the home later pleaded guilty to a health and safety offence and were fined £125,000.

Ms Voisin said the conduct of senior managers at Alexandra House in failing to understand the danger Conroy posed to others amounted to a “gross breach of duty”.

“This I consider to be reckless conduct at senior management level,” she said.

“The support plan and risk assessment drafted by Alexandra House’s senior management was not sufficient to protect Melissa.

“The risk was, and should have been obvious, to Alexandra House from the school care plan and risk assessment, the numerous meetings before Jason’s placement began, the Grant report, the concerns raised by staff and the concerns raised by Melissa herself, who was frightened of him.

“Based on the evidence, it was the gross breach of Alexandra House’s duty of care to Melissa that caused her death, as it allowed a known dangerous young man who was under no supervision at the time the opportunity to murder her.”

In a narrative conclusion, the coroner said Miss Mathieson “died as a result of unlawful killing caused by both the act of strangulation and also due to the acts and omissions by the home entrusted with her care”.

“The home failed Melissa in numerous ways,” she said.

“The resident who went on to strangle her, should not have been placed in the same facility as Melissa at all based on his known risks.

“The decision was wrongly made to place him in the same facility with an ineffective care plan and risk assessment, with staff that were not trained on his level of risk, and managers who failed to act when concerns were highlighted by staff and Melissa.”

Support workers at the home told the inquest they did not know about his history of predatory sexual behaviour.

Several staff told the hearing they would never have been alone with Conroy if they knew of the risk he posed to others, and described reporting their concerns to supervisors.

Miss Mathieson, who was from Windsor, Berkshire, had been sent to Alexandra House by social services when she turned 18, having spent the previous two years in a series of different placements.

In a letter written before her death, Miss Mathieson blamed social services for “destroying” her life.

Titled Social Services Have Destroyed My Life, she wrote: “This was a terrible time for me, and I felt like I was dragged away from my home and everything I knew and being completely disrupted.

“They never took into consideration my age or the distance from my family. I was still a child, just very confused.”

The letter was read to the court as part of the written evidence of Miss Mathieson’s mother, Karen, who died from cancer a year after her daughter’s death.

Mrs Mathieson said “the system” had failed her daughter and had also failed Conroy.

“Not only has Jason Conroy ended Melissa’s life, but he has also ended his own. He should have had his problems addressed a long time ago,” she said.

“The system has failed him, and it has failed Melissa.

“We question the professionals who have had the responsibility for his care, as much as we do of those who had a responsibility for Melissa’s care.”

Miss Mathieson’s father, James, said he felt “helpless” because he and his wife had not been listened to by social services.

“Karen and I blame Jason Conroy for our daughter’s death,” he said.

“But at the same time, how can we blame him when he had said what he was going to do to professionals and was allowed to carry it out.

“We blame the people who put him there with Melissa.”