The man who shot dead a French policeman in a suspected Islamist militant attack had served time for previous armed assaults on law enforcement officers, police sources said on Friday, as authorities sought possible accomplices.The gunman, identified as Karim Cheurfi, opened fire on a police vehicle parked on the Champs Elysees in Paris late on Thursday, killing one officer and injuring two others before being shot dead.The attack, which was claimed by Islamic State, overshadowed the last day of campaigning for Sunday's presidential election first round. Cheurfi, 39, a French national who lived with his mother in the eastern Paris suburb of Chelles, had been convicted for previous gun attacks on law enforcement officers going back 16 years.Police found a note defending Islamic State near his body and believe he had "opened fire on the officers in the knowledge he would be killed by them," a source close to the investigation said.In addition to the assault rifle used in the attack, a pump action shotgun and knives were in his car, the police sources said. Three of his family members have been placed in detention, the French interior ministry announced on Friday.Cheurfi served 10 years in prison after firing on two plainclothes officers in 2001 as they tried to apprehend him in a stolen car. While in detention, he shot and wounded a prison officer after seizing his gun. Released on probation in 2015 from a further two-year jail term imposed for lesser offenses, Cheurfi was arrested again in February after threatening to kill police officers - but released for lack of evidence.A French interior ministry spokesman initially confirmed on Friday that a second man was being sought, based on information from Belgian security services."It's too early to say how or whether he was connected to what happened on the Champs Elysees," ministry spokesman Pierre-Henry Brandet said. "There are a certain number of leads to check. We are not ruling anything out." CONFUSION OVER ISLAMIC STATE CLAIMA potential second suspect was identified as Youssouf El Osri in a document seen by Reuters. Belgian security officials had warned French counterparts before the attack that El Osri was a "very dangerous individual en route to France" aboard the Thalys high-speed train.The warning was circulated more widely among French security services in the hour following the Champs Elysees attack.Brandet later told BFM TV that a man with that name had turned himself in at a police station in Antwerp.Islamic State, which has hundreds of French-speaking fighters, claimed responsibility for the Champs Elysees shooting soon afterwards, in a statement identifying the attacker as "Abu Yousif al-Belgiki (the Belgian)." El Osri's connection with either the downed assailant or the man named by Islamic State remained unclear on Friday. "We don't understand why Islamic State has identified the wrong person," said a police source. "What does seem clear is that Islamic State was planning something." Coming just days after police said they had foiled another planned Islamist attack, arresting two men in the southern city of Marseille, the Champs Elysees shooting dominated the final day of election campaigning.Conservative candidate Francois Fillon and Marine Le Pen, leader of the far-right National Front, both talked tough on law and order while centrist front-runner Emmanuel Macron stressed he was also up to the challenge.