As noted in this space before, the task confronting the Connecticut Department of Children and Families is arguably the toughest in state government. It’s charge is to protect children from social disintegration.

Every day its social workers work to protect children in families with addicted parents, or with single moms depressed or angry with their lot in life, or with parents so emotionally damaged themselves they cannot properly care for children.

There is no margin for error. When DCF gets it wrong, children can die.

So we can sympathize with DCF Commissioner Joette Katz when, responding to a recent tragic outcome, she noted, “Such tragedies are rare among the tens of thousands of families and cases we serve.”

Yet a 2014 incident, as investigated by Child Advocate Sarah Eagan, contains disturbing facts showing that DCF workers missed warning signs and failed to follow policies.

Two-year-old Londyn Raine Sack died of an overdose of her mother’s prescription drug in October 2014, according to Plymouth police. Police had asked DCF to investigate reports of abuse in the family less than a month before the child’s death. Police arrested the mother, Rebekah Robinson, 32, in June on charges of manslaughter and multiple counts of risk of injury. DCF removed the three other children from the home.

Ms. Eagan found that a dozen caseworkers were involved in Ms. Robinson’s case in 2013 and 2014, yet despite numerous warning signs continued to classify the home as safe. Ms. Robinson had complaints lodged against her in 2007 and 2008 for allegedly neglecting the older children in her home.

She then moved to North Carolina, where officials there received multiple complaints against the mother for alleged child neglect and filthy conditions in the home. An adult died of a drug overdose in the home.

Prior to the toddler’s death, DCF did not seek the child-welfare records from North Carolina or the mother’s mental-health records, actions the department’s policies called for.

The proper approach could have led to the earlier removal of the children from a dangerous situation and saved a child’s life.

“We are always working to prevent these types of heartbreaking events from occurring again,” said Ms. Katz. While supervisors in the office were reassigned, no one was disciplined.

That’s not good enough. The commissioner has to make it clear in her words and actions that she will not tolerate incompetence or the failure to follow policy. This death was avoidable, which is the most tragic fact of all.