THE 911 call about a child left alone in a car outside a Germantown townhouse came at 10:15 p.m on Jan. 16. Next was a call the following morning about an open car door and a bloody knife, which led to the grisly discovery of a 1-year-old boy and a 2-year-old girl stabbed to death and two other children injured. Now come the inevitable questions about whether Montgomery County authorities did everything they could to prevent the tragedy.

“I can tell you that was the first question we asked: ‘Did we do everything we could have?’,” Montgomery Police Chief J. Thomas Manger said of the events surrounding the deaths of Norell and Zyana Harris. Their mother, Zakieya L. Avery, and a friend, Monifa Sanford, have been charged after they told authorities they were trying to exorcise the children of “demons.” State’s Attorney John McCarthy asked a county court to order mental health evaluations to determine whether the women are competent to stand trial.

Chief Manger told us that police had no legal foundation to enter the home on the night of the 911 call. The child who had been the subject of the call had been taken inside, no one answered the door and officers, who spent about 40 minutes on the scene, didn’t see or hear anything suspicious. Children’s Protective Services was notified but, with no previous complaints of abuse or neglect involving the children and police reporting that the child had been taken inside by an adult, the agency said it would follow up the next morning.

It appears that the family was receiving assistance from county social services, and Ms. Avery had come to the attention of the court system. Mr. McCarthy told the court that Ms. Avery had a history of mental illness that included being involuntarily committed. What needs examination — and county officials say a review is customary after a child fatality from neglect or abuse — is whether there were missed signals and whether points in the system need to be tightened.

One lesson to be taken from these awful events is the importance of citizens acting on their instincts when they think something might be wrong. Hats off to the neighbor who considered it his business to get involved when he saw a young child alone in a car. And if the woman who noticed the open car door and the bloody knife hadn’t called 911 when she did, there might have been four, not two, children being mourned.