A 59-year-old man has been placed under formal investigation for assassinat, the French charge for murder with premeditation, and remanded in pre-trial detention, the Nice prosecutor's office announced on Thursday. He is accused of killing his 17-year-old son in the family flat in Nice on Monday 3 August. He admitted the killing in police custody and described it as a moment of madness. He had no prior record. He has not been tried, and the presumption of innocence applies.

The sequence, as set out by prosecutors and reported by AFP, is short. On Monday afternoon the man telephoned police to say he had killed his son and intended to take his own life. Officers found the teenager dead at the address, and the father injured after attempting to kill himself. Minutes before their arrival he had sent images of the scene to his wife and to her parents. He told investigators that he had understood that morning that his wife would not be returning home. She had begun divorce proceedings, and the prosecutor's office states that he had not accepted the separation.

The detail that will matter in the investigation is where certain materials at the scene came from. The man works as a care assistant in a hospital, and he told investigators he had taken them from his workplace. Whatever weight a court eventually gives that, it is the reason the charge is assassinat rather than meurtre. French law distinguishes the two by premeditation, and the difference is not academic: the maximum sentence rises from thirty years to life. His own account of a sudden loss of control will be tested against the preparation the file appears to describe.

Three days before the killing, on the Saturday, the mother had left the family home and gone to a police station in Nice to file a complaint against her partner for domestic violence and harassment. That is the part of this case that is not particular to it.

The pattern is documented and it is well known to the services that deal with it. The period immediately around a separation is the most dangerous point in a violent relationship, because it is the moment the abuser loses control of the person leaving. Where children are involved, the risk does not stay confined to the partner. Killing a child is understood in that literature as an act directed at the mother, which is why the systems built to respond to domestic violence in France, the protection order, the emergency phone, the eviction of the violent partner from the home, are all supposed to be triggered by exactly the event that occurred here, a complaint filed by a woman who has just left.

What has not been established, and what an investigating judge will now have to determine, is what happened to that complaint between Saturday and Monday. Whether it was assessed for danger, whether the children were considered in that assessment, and whether anything was set in motion before the weekend ended, are questions the file will have to answer. They are the only questions in this case whose answers could change anything for anybody else.

In France, 3919 is the national helpline for women who are victims of violence, free and confidential. 119 is the national number for children in danger. In an emergency, call 17 or 112.