Motive for five homicides remains unclear; suspect to undergo psychiatric examination.
The mother, 35, and two daughters, aged seven and 10 years, were stabbed several times overnight from Sunday to Monday, prosecutor Jean-Baptiste Bladier said on Tuesday.
The two younger sons, aged nine months and four years, were either suffocated or drowned, Bladier said.
Police broke in through a bedroom window of their ground-floor flat to find a “very violent crime scene” covered in blood, the prosecutor said.
The bodies were found on Monday night in an apartment in the town of Meaux, 40km (25 miles) east of Paris after friends and family sounded the alarm.
The motive for the killings was unclear.
The suspect was arrested on Tuesday outside his father’s home northeast of the French capital, the prosecutor said.
He told police “he knew why he had been detained, and spoke of his unhappiness and depression”, Bladier said. He was receiving medical care for wounds to his hand.
The suspect had stabbed his partner once before when she was pregnant with their older son in 2019, but the investigation was dropped because he was declared mentally unsound at the time of the attack, the prosecutor said. He had been placed in a psychiatric hospital in 2017 and also attempted suicide that year.
The couple had been together for 14 years and had known each other since high school, Bladier said. They had their four children before they married in October.
Neighbours told investigators they had heard screams during the early hours of Christmas Day. The alarm was raised later in the day after they failed to get any response from the family.
None of the family members’ names were released, according to French law protecting minors who are victims of crimes.
Authorities are opening an investigation into five homicides, and the suspect will undergo psychiatric examination to determine the next steps, the prosecutor said.
On average, a woman is killed every three days in France. Some 118 women were killed by their partner or ex-partner in France last year.