A judge has declared a mistrial in the closely watched case of a Massachusetts mother accused of killing her three young children, after jurors failed to agree whether she was criminally responsible for their deaths.

The decision came on Friday after the jury deliberated for more than 38 hours across seven days in the trial of Lindsay Clancy.

Clancy, a former labour and delivery nurse, admitted to strangling five-year-old Cora, three-year-old Dawson and eight-month-old Callan at their Duxbury home in January 2023. She then jumped from a second-floor window in an apparent suicide attempt, leaving her paralysed.

She pleaded not guilty by reason of insanity, known in Massachusetts as a lack of criminal responsibility.

Her lawyers argued that she was suffering from postpartum psychosis, compounded by an undiagnosed bipolar disorder and a succession of psychiatric medications. Prosecutors maintained that Clancy understood her actions and deliberately planned the killings.

The competing accounts have made the case deeply polarising across the United States.

For some, Clancy’s case represents a catastrophic failure of the maternal mental-health system. Evidence showed that she repeatedly sought treatment and had been prescribed 13 psychiatric medications in the months before the killings. Her supporters argue that her condition, rather than criminal intent, drove her actions.

Others reject the suggestion that mental illness absolved her of responsibility, pointing to evidence prosecutors said demonstrated planning and an awareness that the killings were wrong. The debate has also raised concerns among mental-health advocates that linking postpartum illness primarily with violence could further stigmatise mothers seeking help.

That division was ultimately reflected in the jury room. Clancy’s defence said 11 jurors favoured acquittal on the grounds that she was not criminally responsible, while one held out. The court did not independently confirm the split.

Judge William Sullivan declared the mistrial after concluding that further deliberations were unlikely to produce a unanimous decision.

Clancy remains charged with three counts each of murder and strangulation and is being held at a psychiatric hospital.

Prosecutors have not yet decided whether to retry her, offer a plea agreement or drop the charges. The case is due to return to court on September 29.