A note expressing support for Isis has been found near the body of a gunman who shot a police officer dead in the centre of Paris.

Sources within the investigation told news agencies of the discovery as a probe continued into the chief suspect, a 39-year-old Frenchman who was the subject of a previous terror investigation.

Police also found a pump-action shotgun and knives in his car, showing the attacker was armed for an extended assault.

Isis swiftly claimed responsibility for the attack, giving the perpetrator a war name indicating he had links in Belgium and hailing him as a "fighter of the Islamic State".

The speed of the claim and the fact the attacker was identified suggests he was in contact with the group and informed militants of his plan in advance.

War names were previously used by Isis to identify the jihadis who carried out the Paris and Brussels attacks, which were coordinated with commanders in Syria

The group has issued detailed instructions on carrying out car and shooting attacks in its propaganda magazines, which urged followers to leave letters or other proof of their support to be found by authorities.

Gunman attack on the Champs-Elysees Show all 23 1 /23 Gunman attack on the Champs-Elysees Gunman attack on the Champs-Elysees Police officers secure the area after a gunman opened fire on Champs Elysees. One police officer has been killed, and a second injured by a gunman on The Champs Elysees Getty Images Gunman attack on the Champs-Elysees Police officers block the access of a street near the Champs Elysees in Paris after a shooting in which one police officer was killed and another wounded. The attacker was killed in the incident on the world famous boulevard that is popular with tourists Getty Images Gunman attack on the Champs-Elysees Police officers control passersby near the Champs Elysees in Paris after a shooting. One police officer was killed and another wounded today in a shooting on Paris's Champs Elysees, police said just days ahead of France's presidential election. The attacker was killed in the incident on the world famous boulevard that is popular with tourists. Getty Gunman attack on the Champs-Elysees People run away after a shooting at the Champs Elysees in Paris Getty Images Gunman attack on the Champs-Elysees French people hold their hands in the air after a shooting in which a police officer was killed along with the attacker and another police officer wounded in an attack on the Champs Elysees in Paris EPA Gunman attack on the Champs-Elysees Bystanders are frisked by police officers near the Champs Elysees avenue in Paris, France, after a fatal shooting in which a police officer was killed along with an attacker AP Gunman attack on the Champs-Elysees People hold their hands up as they walk towards police officers before being controlled near the site of a shooting at the Champs Elysees in Paris. France's interior ministry said the attacker was killed after opening fire on police in the early evening on the world-famous boulevard Getty Images Gunman attack on the Champs-Elysees Police look through binoculars as officers secure the area after a gunman opened fire on Champs Elysees Getty Images Gunman attack on the Champs-Elysees A tourist is blocked by French soldiers next to the Champs Elysee Avenue during ongoing police operations after a shooting in which two police officers were killed along with their attacker and another police officer wounded in a terror attack near the Champs Elysees in Paris, France EPA Gunman attack on the Champs-Elysees A tourist is blocked by French soldiers next to the Champs Elysee Avenue during ongoing police operations after a shooting in which two police officers were killed along with their attacker and another police officer wounded in a terror attack near the Champs Elysees in Paris, France EPA Gunman attack on the Champs-Elysees French Police officers, after a shooting in which two police officer were killed along with their attacker and another police officer wounded in an attack near the Champs Elysees in Paris, France EPA Gunman attack on the Champs-Elysees Forensic experts and police officers examine evidence from a police van on the Champs Elysees in Paris, France, after a fatal shooting in which a police officer was killed along with an attacker AP Gunman attack on the Champs-Elysees Police investigators inspect the car used by the attacker on the Champs Elysees Avenue after a policeman was killed and two others were wounded in a shooting incident in Paris, France Reuters Gunman attack on the Champs-Elysees A French soldier patrols on the Champs Elysees in Paris after a shooting. One police officer was killed and another wounded today in a shooting on Paris's Champs Elysees, police said just days ahead of France's presidential election. France's interior ministry said the attacker was killed in the incident on the world famous boulevard that is popular with tourists Getty Images Gunman attack on the Champs-Elysees Police officers patrol near the site of a shooting at the Champs Elysees in Paris Getty Images Gunman attack on the Champs-Elysees French military secure the area after a gunman opened fire on Champs Elysees Getty Images Gunman attack on the Champs-Elysees French military secure the area after a gunman opened fire on Champs Elysees Getty Images Gunman attack on the Champs-Elysees Police secure the area after a gunman opened fire on Champs Elysees in Paris, France Getty Images Gunman attack on the Champs-Elysees One police officer has been killed, and a second injured by a gunman on The Champs Elysees Getty Gunman attack on the Champs-Elysees Police officers secure the area after a gunman opened fire on Champs Elysees Getty Images Gunman attack on the Champs-Elysees Police secure the area after a gunman opened fire on Champs Elysees. Getty Images Gunman attack on the Champs-Elysees French Prime Minister Bernard Cazeneuve, flanked by Interior Minister Mathias Fekl and Justice Minister Jean-Jacques Urvoas, speaks to the media outside the Elysee Palace after a defense council meeting in Paris, France EPA Gunman attack on the Champs-Elysees A damaged window is pictured on the Champs Elysees boulevard in Paris AP

“Lest the operation be mistaken for one of the many random acts of violence that plague the West, it is essential to leave some kind of evidence or insignia identifying the motive and allegiance to the caliphate, even if it is something as simple as a note,” said an article calling for knife attacks in October.

The article, followed by another specifically calling for lorry attacks, was followed by massacres in Berlin, Stockholm and London.

Notes claiming to express support for Isis were found following bombings targeting the Borussia Dortmund football team earlier this month, although they later emerged to be fabrications by a market trader hoping to profit from falling shares.

The main suspect for Thursday's atrocity is Karim Cheurfi, who had an extensive criminal record including the attempted murder of two police officers.

Three of his relatives were detained in searches of his family home in Chelles, although police stressed the move was standard procedure for questioning.

Cheurfi was jailed for attempted murders that saw him shoot two police officers – one during a car chase in 2001 and a second days later after seizing his gun in custody, Le Parisien reported.

At a trial in 2003, Cheurfi was sentenced to 20 years imprisonment, which was later reduced to 15 on appeal, and it was unclear when he was released.

Police officers search the house of a suspect after a gunman opened fire on police in the Champs Elysees, in Chelles on 20 April (AFP/Getty Images)

He was also arrested in February as part of a counter-terror investigation - reportedly for threatening police officers - but was released by the courts because of insufficient evidence the following day.

Pierre-Henry Brandet, a spokesperson for the French interior ministry, described the suspected gunman, who has not yet been officially named, as an “individual known by the judiciary and police services – a dangerous individual”.

French authorities said they would not formally name the gunman, who was shot dead at the scene, until they had established whether he was working with accomplices.

Witnesses said he parked a car behind a police van on the Champs Elysees on Thursday night, emerging to open fire with a Kalashnikov.

One officer was killed and two more injured, as well as a German woman whose foot was injured.

One of the officers underwent emergency surgery overnight, but an interior ministry official said both injured officers were now “out of danger”.

It was one of almost a dozen attacks and plots targeting the French security services since 2012, including the murder of a police officer and his wife by an Isis supporter north of Paris, a machete attack outside Le Louvre and an attempted murder at Orly airport.

The latest killing came just days before the presidential election, following an operation to foil an “imminent” Isis bombing plot that was in an advanced stage of preparation.