The mother of a man suspected of stabbing a German tourist to death near the Eiffel Tower in Paris reported concerns about her son weeks before the attack, a prosecutor has said.
Two others, including a British man, were injured in the attack on Saturday.
A 26-year-old French man was later arrested, and anti-terrorism prosecutors opened an investigation.
France's interior minister said there was a "failure" in the alleged attacker's psychiatric treatment.
Gérald Darmanin told BFM TV that the attacker had an "acute mental illness", adding that "doctors said on several occasions that he was doing better, was more normal and could be free".
Prosecutor Jean-François Ricard said the suspect had pledged allegiance to the Islamic State group (IS).
Armand R, a French national born in France to Iranian parents, converted to Islam aged 18, the prosecutor said. He was imprisoned in 2016 for four years after making plans to travel to Syria to join IS.
Mr Ricard said that the suspect had been under surveillance for suspected extremism and that three people, including members of his family, had been detained for questioning.
He told reporters that the alleged attacker's mother "reported concerns about her son's behaviour" in late October "as he had turned in on himself".
However, there was not enough information to allow for a new prosecution, he added.
Police said the suspect was supposed to be following treatment for psychiatric problems.
The victim killed in the attack was a German tourist who worked as a nurse.
Mr Darmanin said the victim was with his wife when he was attacked and fatally stabbed on Quai de Grenelle.
He said the wife's life was saved by the intervention of a taxi driver and that the suspect fled across a nearby bridge spanning the River Seine.
After crossing to the north side of the river he attacked two more people: a French man aged around 60 and a 66-year-old British tourist who was hit in the eye with a hammer.
The suspect was then Tasered by police and arrested on suspicion of assassination - defined in French law as premeditated murder - and attempted assassination in relation to a terrorist enterprise.
The two people injured were treated by emergency services and on Sunday, Health Minister Aurélien Rousseau told French media they were "in good health".
Video published online appeared to show the moment the suspect was apprehended by armed police not far from where the attack happened.
Mr Darmanin said the alleged attacker was heard shouting "Allahu Akbar", Arabic for "God is greatest", and told police he was upset because "so many Muslims are dying in Afghanistan and in Palestine".
The suspect is also understood to have suggested France was complicit in the deaths of Palestinians in Gaza.
