A policeman was shot dead on the Champs Elysees in Paris on Thursday night by a gunman who was under preliminary investigation for terrorism but "let go" in an attack claimed by the Islamic State of Iraq and the Levant.

Isil claimed the shooting, days before French presidential elections, via its Amaq news agency, naming the attacker as Abu Yousif al-Bajiki (the Belgian), but police said the identity had not been confirmed.

President Francois Hollande said he was convinced it was a terrorist attack.

The gunman had recently been detained for questioning in a police station in Meaux, outside Paris, after "informants" had indicated that he was "seeking to obtain weapons to kill policemen", according to reports.

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But due to a lack of evidence, anti-terror prosecutors "let him go".

The attacker jumped out of a car around 9pm local time and opened fire on a police car parked on the Champs-Elysees, hitting at least two officers. The man's gun, thought to have been a Kalashnikov, was described by authorities as a "war weapon".

The man was then shot dead as he fled and police later carried out searches at an address east of Paris, Pierre-Henry Brandet, the interior ministry spokesman, confirmed.

The 39-year old gunman, was known by intelligence services for radicalisation and had previously targeted police 16 years ago in a car chase, when he opened fire on officers, according to BFMTV.

Le Parisien reported that authorities know him as Karim C. The prosecutor has declined to name him, although he confirmed they did know his identity

During questioning in a police station, he managed to get hold of a police weapon and fire five times. The policeman survived, but the man was sentenced to several years in prison for the attack.

Mr Brandet said police had a name for the gunman and it matched a suspect known to security services but the attacker's fingerprints had not been matched with those of the suspect.

French prosecutors said they had opened a counterterrorism investigation, while a French interior ministry spokesman said there may be more than one attacker and the police had been "deliberately targeted".

According to BFM TV, the attacker had boasted of wanting to kill police on the Telegram messaging service.

There were unconfirmed reports last night that an arrest warrant had been issued for a second suspect who had arrived in Paris from Belgium.

This story was orginally published by The Daily Telegraph UK.