PARIS — At least four police officers were killed on Thursday at Paris Police Headquarters by a man with a knife, according to a French police union official.

The official from the Alliance Police Nationale union, Loïc Travers, told reporters that the assailant was an administrative employee who had worked at Police Headquarters for more than 20 years.

The unidentified attacker was shot and killed by a police officer, Mr. Travers said.

He said the assailant appeared to have started attacking officers in his office before moving to other areas in the headquarters.

“The motive is not yet known,” Mr. Travers added.

The headquarters of the Paris police prefecture in central Paris, near Notre Dame, did not immediately respond to requests for comment. The Paris prosecutor was on the scene, his office said, giving no further information.

The severity of the assault will most likely rekindle concerns about how the French capital has become a target of terrorists, most notably with the attacks on Charlie Hebdo in 2015 and a series of coordinated attacks in a nighttime rampage across the city later that year that left more than 100 dead.

In 2017, a veteran police officer, Xavier Jugelé, was shot and killed during a terrorist attack on the Champs-Élysées.