A man stabbed his mother and sister to death and seriously injured another person in a town near Paris on Thursday before being shot dead by police.
The 36-year-old had serious mental health problems and had been on a terror watch list since 2016, Interior Minister Gerard Collomb told reporters after visiting the scene in the town of Trappes.
The motive for the violence remained unclear despite a claim by the Islamic State (IS) group that it was an attack by one of its fighters responding to the terror organisation's propaganda.
Collomb said the attacker was "mentally unstable, rather than someone who was committed, who would respond to orders from a terrorist organisation".
The Versailles public prosecutor, however, said in a statement late Thursday that the assailant had shouted "Allahu Akbar" (God is Greatest) and threatened police before he was shot.
"Investigations are under way to determine the circumstances and the exact sequence of events as well as the motivation of the perpetrator, which is still uncertain at present," the statement added.
Experts have urged caution about some recent IS claims taking responsibility for attacks around the world, as governments and experts have thrown several such declarations into doubt.
The attacker's third victim, 66, was in a critical condition after being airlifted to hospital, officials said.
- 'A guy who had lost it' -
The attack happened around 9 am (0700 GMT) in Trappes, a poor town with a large Muslim population about 30 kilometres (20 miles) southwest of the capital.
Armed police responded to reports of violence and were threatened by the man who had stabbed his 71-year-old mother to death in her home, Collomb added. The man's sister was aged 49.
"He walked towards the police with his knife," he said. "Police then opened fire."
The neighbourhood, a leafy area of detached homes close to high-rise concrete apartment blocks, remained partially sealed off by police Thursday afternoon.
Locals said the killer, who has not yet been named, was separated from his wife and had not seen his children in a long time, adding that he had worked as a bus driver and was living with his mother.
"I had coffee with him often but I hadn't seen him for a month," said one neighbour, a 59-year-old who gave his name as Pascal.
"They're talking about terrorism but this wasn't a terrorist, it was a guy who had lost the plot," he added.
The knifeman had been on the terror watch list since 2016 after expressing extremist views, a security source told AFP.
BFM television reported that he had serious alcohol and drug problems.
- 'People think it's Baghdad' -
Just a short drive from the wealthy area of Versailles, home to the world-famous palace, Trappes is known for social problems linked to poverty, drugs and hardline interpretations of Islam.
About 50 locals are suspected of having left France to fight for IS in Syria and Iraq.
Residents expressed concern that the killings would reinforce the area's negative image, which persists despite the success of a few home-grown stars, including actors Omar Sy and Jamel Debbouze.
"People think it's Baghdad around here -- my colleagues don't want to drop me off after work," said Anis Fillali, a 21-year-old salesman standing at the police cordon.
France remains on high alert after a string of jihadist attacks since 2015 that have claimed the lives of 246 people.
On its propaganda channel Amaq, IS claimed the Trappes killer "carried out the attack in response to calls to target subjects of the countries of the coalition" fighting the jihadists.
The group claimed responsibility for a massacre in Las Vegas last year that saw a wealthy accountant kill 58 concert-goers in the deadliest mass shooting in recent US history.
But US investigators concluded there was "no evidence of radicalisation or ideology to support any theory that (Stephen) Paddock supported or followed any hate group or any domestic or foreign terrorist organisation".
French terror expert Jean-Charles Brisard, head of the Center for the Analysis of Terrorism (CAT), said it was "too soon to say" if IS had a hand in the violence in Trappes.
"But remember that in 2017 we had three claims of responsibility that were outlandish by Islamic State... above all the massacre in Las Vegas," he told AFP.