As Fiona Anderson wandered the streets of Lowestoft before dawn on Monday, it would have been difficult for her to pass unnoticed. Heavily pregnant and wearing a long white coat, she was clutching a teddy bear as she traversed the Suffolk port for hours on end.

Within hours, the body of the 23-year-old mother was found outside a multi-storey car park in the town with the teddy bear still beside her. Fiona’s unborn daughter, whom she had named Evalie, was also killed in the fall; in their nearby home her three young children lay dead.

Suffolk Police have reiterated its call for witnesses to help piece together Fiona’s last moments. New CCTV images show the young woman leaving the keys to her home at a shelter where she believed her estranged partner, Craig McClelland, was staying.

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Barely 90 minutes later, Fiona, who is believed to have attacked Mr McClelland with a knife the night before her death, was dead from multiple head injuries. The bodies of her children were discovered by officers who forced their way into her rented flat. It is likely the young mother had left the keys intending that her ex-boyfriend find his dead children.

Despite contact with social services in recent weeks and a visit to her home by police investigating Mr McClelland’s stabbing a few hours before her death, the authorities were unable to intervene to prevent the tragedy.

As far as her family and friends are concerned, the events that led to such a grim tragedy are clear – a history of mental illness and an inability to cope with the obstacles that life had thrown at her by “caring too much”.

In a statement, her parents Michael and Kerry, and her sisters Claire and Laura, said: “Fiona was a beautiful, intelligent girl and a loving and caring person, but she had suffered from mental illness since she was young and we believe she was driven to her actions.

“This was not our Fiona – she was not herself. She cared passionately about others but often brought stress on herself by caring too much. She was gentle but ended up under too much pressure.

“As a family we were close but she would often push us away, keen to do her own thing and not to listen when we offered support. We wish more had been done to recognise her mental health problems, which could be masked by her intelligence and creativity.”

While her history of problems is understood, what specific motives drove Fiona, a loving mother who had enjoyed making costumes for her young offspring, to act as she did is a question still engulfing those who loved her. Friends said she had contacted social services for help but became worried that her children would be removed from her.

Post-mortem examinations on the children have so far failed to pinpoint a cause of death and tests are ongoing. A friend, who asked not be named, said a Facebook tribute page to Fiona had been shut down because of abusive messages.

Her relationship with Mr McClelland, 24, had broken down a fortnight ago amid accusations that he had a new girlfriend.

The recriminations from the break-up reached a peak in the early hours of Monday when police were called to Fiona’s flat in the south of the town following reports that a man – now known to be Mr McClelland – had been stabbed. It is understood Mr McClelland, who broke down in the arms of his mother when he visited the scene of his children’s deaths, was treated at hospital for a minor wound.

Suffolk Police confirmed that officers had spoken over an intercom to Fiona, who refused to allow them into the property, and they left, adding that it had referred the case to the Independent Police Complaints Commission.