A 45-year-old technology administrator at the police headquarters in central Paris went on a knife rampage inside the building on Thursday, killing three police officers and an administrative worker before he was shot dead by an officer, French officials said.
French broadcaster BFM TV said the attacker had converted to Islam 18 months ago.
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Officials did not say anything about the motive for the attack and said they were still trying to discover if there was a terrorism link.
The man launched the attack in his office then moved to other parts of the large 19th-century building across the street from the Notre Dame Cathedral.
An officer stopped the attack when he shot the assailant in the compound's courtyard, said a police official. The official was not authorized to talk publicly about the case and requested anonymity.
Emery Siamandi, who works at police headquarters, said he was in the stairwell leading to the chief's office when he heard gunshots.
"I told myself, this isn't right," Siamandi said. "Moments later, I saw three policewomen crying. I couldn't help them in any way, and their colleagues were crying, too, so I figured it must be serious."
He said he saw one officer on his knees in tears.
The area around the police headquarters was sealed off and a metro station was shut for security reasons as the attack unfolded.
The dead were three men and a woman, according to Paris prosecutor Remy Heitz. Another person was wounded and is undergoing surgery.
Heitz said police were searching the attacker's home, and that anti-terrorist investigators were evaluating what had happened, for any terrorist links. The attacker's wife had been brought into police custody but not charged, the Paris prosecutor's office said.
"It's the worst scenario possible, an internal attack with colleagues working together," said Philippe Capon of the UNSA police union.
Capon cautioned against jumping to conclusions on the motive and said, "Nothing can be ruled out, including a personal issue."
French media reported the employee carried out the attack with a ceramic knife.
President Emmanuel Macron stopped by police headquarters to show solidarity with officers and department employees, his office said.
The neighborhood where the police compound is located, a busy tourist destination, was locked down, the Cite metro stop was closed and the bridge between Notre Dame and the headquarters building was blocked off.
"Paris weeps for its own this afternoon after this terrifying attack in the police headquarters. The toll is heavy, several officers lost their lives," Mayor Anne Hidalgo tweeted.
Speaking outside the police headquarters, French Interior Minister Christophe Castaner said the attacker was known to his colleagues and had worked for some time in the IT department.
"He had never presented any behavioral issues, he had never presented the slightest cause for alarm before going on his deadly rampage today," Castaner said. "There were no warning signs."
A police official and member of the collective "Police up in Anger," which lobbies for better conditions for officers, was quoted by France Info radio as saying the assailant had experienced issues with his supervisor.
"I know there were tensions between him and his direct supervisor," Christophe Crepin told France Info. "I do not think this is a terrorist act."
Jean-Marc Bailleul, a police union leader, described the incident as criminal rather than an act of terror. "It was a moment of madness," Bailleul told BFM TV.
In the past four years, the French capital has been rocked by violent attacks resulting in mass casualties.
Coordinated bombings and shootings by Islamist terrorists in November 2015, at the Bataclan theater and other locations around Paris, killed 130 people in the deadliest attacks in France since World War II.
In a three-day killing spree in January 2015, Islamist gunmen killed reporters and illustrators at satirical weekly Charlie Hebdo, police officers, and shoppers at a Jewish supermarket.
