A veteran police employee in France slipped a knife through security at the heavily guarded Police Headquarters in the heart of Paris on Thursday, killing four of his colleagues before being shot dead in the building's vast courtyard, reports the New York Times.

Three of the people he killed were police officers.

Christophe Crépin, a spokesperson for the French police union, said earlier, along with two anonymous police sources, that four police officers had been killed. But Paris prosecutor Remy Heitz later said that three police officers had been killed as well as an administrative worker, reports NBC News.

The employee, a 45-year-old man who worked in computer services for the intelligence division of the Paris police, moved methodically from his office, up the stairs and back down, killing one woman and three men as he went, police union officials told French television.

Discontent among police officers was already rising before the attack. The unhappiness and disquiet plaguing the French police have led to a record number of suicides and to a mass demonstration by officers through Paris on Wednesday, the biggest in nearly 20 years, the New York Times reports.

"It might be the expression of a new malaise at the heart of the police, like this wave of suicides," Denis Jacob, a police union official, told French television on Thursday after the attack. An official with another police union said there were morale problems among administrative workers, who were held in low esteem.

The attacker had converted to Islam, according to a police spokesman, who spoke on the condition of anonymity in accordance with police protocols. The spokesman added that that was just one element of the investigation, and that his conversion "doesn't mean that he is radicalized."

The suspect, a 20-year-veteran of the force, was not immediately identified by the police. He had not previously exhibited signs of trouble, officials said.