An employee armed with a knife attacked officers inside Paris police headquarters Thursday, killing at least four before he was fatally shot, a French police union official said.

Police union official Loic Travers told reporters the attack appears to have begun in an office and continued elsewhere in the large police compound across the street from Notre Dame Cathedral.

The number of people injured was not immediately known.

Travers said the motive is unknown, but that the 20-year police employee allegedly responsible for the attack worked as an administrator in the intelligence unit and had not posed known problems until Thursday.

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He said he could not remember an attack of this magnitude against officers.

Emery Siamandi, an employee at police headquarters, said he heard gunshots and immediately saw two officers come outside an office weeping. A third officer, who Siamandi described as the person who shot and killed the assailant, came out on his knees, also in tears.

The attack came a day after thousands of officers marched in Paris to protest low wages, long hours and increasing suicides in their ranks.

France’s prime minister, interior minister and the Paris prosecutor were on their way to the scene.

The Paris prosecutor’s office said no information could be provided at this stage.

The neighborhood, one of Paris’ busiest tourist attractions, was locked down, the Cite metro stop was closed and the bridge between Notre Dame and the headquarters building was blocked off.

Extremists have repeatedly targeted French police in France in recent years. In 2017, a gunman opened fire on the Champs-Elysees boulevard, killing one officer before he was shot to death.

In 2016, an attack inspired by the Islamic State group killed a police officer and his companion, an administrator, at their home in front of their child.

(AP)