By Matthieu Protard

PARIS (Reuters) – Investigators are not ruling out any hypothesis in their probe into the fatal knife attack at the headquarters of the Paris police, authorities said on Friday.

A 45-year-old IT assistant killed three police officers and an administrative worker inside the Paris police building on Thursday before he was shot dead by an officer.

French media reported that the attacker had converted to Islam 18 months ago. Officials have not said anything about a possible motive for the attack and said they were still trying to discover if there was a terrorism link.

“No hypothesis is being ruled out at this stage,” Paris police chief Didier Lallement said at a news conference. He said he would not respond to further questions on the matter.

The attack took place on the historic Ile de la Cite island in the River Seine, close to Notre-Dame de Paris cathedral.

In the past four years, the French capital has been rocked by violent attacks resulting in mass casualties.

Co-ordinated bombings and shootings by Islamist militants in November 2015, at the Bataclan theater and other locations around Paris, killed 130 people in the deadliest attacks in France since World War Two.

(Reporting by Sudip Kar-Gupta, Writing by Matthieu Protard, Editing by Geert De Clercq and Janet Lawrence)