Three more people close to the attacker Cherif Chekatt, who was shot dead by police on Thursday, were still being held, it said.
But his parents and two of his brothers were freed “due to the lack of incriminating evidence at this stage”, the prosecutor's office added.
More than 700 French security forces had been hunting for 29-year-old Chekatt since Tuesday night's bloodshed, in which four people died — the latest in a string of jihadist attacks to rock France.
France's interior minister on Friday dismissed a claim by the Islamic State (IS) group that it was responsible for the attack.
Police have been focusing their investigation on whether Chekatt had any help in carrying out his attack or while on the run.
In an interview broadcast on Saturday evening, a man identified as Cherif Chekatt's father said his son had become a follower of IS.
“He would say Daesh is fighting for a just cause,” the man, named as Abdelkrim Chekatt, told France 2 television, using the Arabic acronym for IS. “I told him 'Forget about Daesh, don't listen to what they say. Don't you see the atrocities they commit?'”
He added that he had had no knowledge of the attack.
“If he had told me about his plans, I would have reported him to the police, that way he would have not killed anyone or been killed himself,” the man said.