A knife-wielding attacker has stabbed a senior police officer to death outside his home in a suburb of Paris.
The attacker and a female companion of the police commander were later found dead after police commandos stormed the home and rescued the couple's three-year-old son.
French officials said anti-terrorism prosecutors were investigating the attack.
The Islamic State's Amaq news agency cited an unnamed "source" as saying an IS fighter carried out the attack, but the extremist jihadist group has not officially claimed responsibility.
The off-duty police commander was attacked outside his home in Magnanville, about 35 miles west of Paris, interior ministry spokesman Pierre-Henry Brandet told reporters at the scene.
The attacker then retreated indoors and elite police commandos laid siege to the residence, eventually storming it after a three-hour stand-off. Mr Brandet said the woman, the commander's companion and a fellow police worker, was found dead, as was the attacker. The police couple's son was unharmed.
Although officials said the attacker was killed by police when they stormed the residence, it was unclear how the woman was killed.
"The toll is a heavy one," Mr Brandet told reporters, his voice heavy with emotion. "This commander, this police officer, was killed by the individual ... (and) we discovered the body of a woman. The assailant, the criminal, was killed. Thankfully, a little boy was saved. He was in the house. He's safe and sound. He was saved by police officers."
The Paris prosecutor's office said anti-terrorism investigators had been brought in to the case given the target, the method behind the attack, and what the attacker said to police during the ensuing stand-off.
The office did not elaborate, but French media, some of them citing unnamed neighbours, reported that the attacker was heard shouting "Allahu Akbar" - Arabic for "God is Great" - during the attack.
French prosecutor Vincent Lesclous - who said he knew the murdered police commander - told reporters the boy was found "shocked but unharmed". He said the assailant's identity was unknown.
If the crime was organised or inspired by IS, it would fit in with a long-established pattern of jihadist violence. France has seen a series of stabbings aimed at police officers or soldiers carried out by Muslim radicals. IS has encouraged its supporters to stage such attacks.
Tensions have been particularly high since IS extremists claimed responsibility for the November 13 Paris attacks that killed 130 people.
President Francois Hollande condemned what he described as an "odious act", saying he would hold a meeting at his Elysee Palace office later.
"Light will be shed on the circumstances of this abominable drama whose investigation, under the authority of justice, will determine the exact nature," he said.