Latest: Note defending Islamic State found next to body of gunman, reports
Update 2.40pm: A legal source claims investigators have found a note defending Islamic State next to the body of the gunman who shot dead a policeman in Paris.
The attacker was killed by security services on the Champs-Elysees last night.
The French president has visited one of the two other police officers who were injured.
Earlier: The gunman who shot and killed a police officer on the Champs-Elysees in Paris was detained in February for threatening police and then freed.
Two French officials said the gunman was detained towards the end of February after speaking threateningly about the police, but he was then released due to a lack of evidence.
He was also convicted in 2003 of attempted murder in the shooting of two police officers.
Police shot and killed the gunman after he opened fire on a police van on Paris' most famous boulevard on Thursday night.
The Islamic State group quickly claimed responsibility for the attack.
The Paris prosecutor's office leading the investigation has said a pump-action shotgun and knives were found in the gunman's car.
Police have detained for questioning three of his family members, as investigators seek to determine whether he was acting alone and where he got his weapons.
A Belgian man who had been linked by some as an accomplice to the gunman has turned himself in, and authorities said there was no link to the incident.
A prosecutor in Antwerp said: "That man came to police after he saw himself appear on social media as terror suspect No. 1 relating to yesterday's facts."
The prosecutor added that the man had nothing to do with the attack. "He was not part of a terrorism investigation."