A knife-wielding man has killed four staff at police headquarters in Paris.The attacker, who has not been named but is also said to be an employee there, was later shot dead by police.There has been no official statement so far. Witnesses described people running everywhere, many in tears. The area in the île de la Cité - in the centre of Paris - has been sealed off.The attack comes a day after police went on strike across France over increasing violence towards officers.It happened at about 13:00 local time (11:00 GMT; 12:00 BST). The attacker is said to have gone into the building and straight to his office where he began attacking colleagues with a knife.President Emmanuel Macron, Prime Minister Edouard Philippe and Interior Minister Christophe Castaner have all gone to the site.Paris Mayor Anne Hidalgo confirmed on Twitter that "several people" had been killed in the attack, which took place near major tourist sites including the Notre-Dame cathedral.Three men and one woman were killed in the attack, according to French broadcaster BFMTV.Another person was also reportedly wounded.According to French media, the attacker was a 45-year-old man who had worked in an administrative capacity for the Paris police force for 20 years.They said he had been working in the police force's intelligence division.There were tensions between the knifeman and his supervisor, according to police union official Christophe Crepin."I do not think this is a terrorist act," Mr Crepin, who knew the attacker, told Franceinfo Radio.Paris police headquarters is located in the heart of ParisBFMTV reports that he stabbed two people inside two offices, one person on a stairway and the fourth person inside the building's courtyard - where the knifeman was shot dead."Police were running around in panic," an eyewitness who was inside the courtyard at the time of the attack told Le Parisien newspaper."I was surprised to hear shooting because this is not a place where you hear that kind of thing. I first thought it was a suicide because there are a lot of those at the moment," he added.The attack follows a rare nationwide police strike on Wednesday, which saw thousands of officers demonstrating in Paris over working hours, shortages in resources and controversial pension reforms.French police get angry over working conditions and suicide ratesPolice unions say there have been more than 50 suicides by police officers since the start of the year.They blame the rising rates on difficult working conditions and increasing violence towards police.