An administrator armed with a knife attacked officers inside Paris Police Headquarters Thursday, killing at least four before he was fatally shot, officials said.

Police union official Loic Travers told reporters the attack appeared to have started 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 clear.

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

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

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.

“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 a department employee carried out the attack with a ceramic knife in a part of the headquarters building that is not open to the public.

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