[9] The attacker was reported as being 45-year-old Mickaël Harpon, an IT specialist who worked in the intelligence unit of the police headquarters for the last 16 years and held a security clearance, giving him access to restricted information like the watchlist of terror suspects, addresses of police officers and data on French citizens and their families who had returned after they fought in the Syrian Civil War.

[10][21] The counter-terrorism head said that the attacker had radical and extremist religious views, that started when he justified some violent acts with an Islamist background, stopped wearing Western clothes and to not talking to women excluding those of his family.

Among those, Harpon wrote "Allah Akbar" and "Follow our much-loved Prophet Muhammad, and meditate on the Quran".

[5] Investigation of Harpon's computer and phone revealed that he searched the internet for "how to kill infidels" shortly before the attack.

[28][29][needs update] France Inter reported in October 2019 that the investigators suspected that the attack was the result of a suicidal delirium, and that the ISIL videos on the USB drive found at Harpon's office belonged to his coworkers.

A day after this statement, when it became public that his coworkers had filed a complaint and that there was no investigation,[10] Castaner said there were "malfunctions" that failed to prevent the attack.