A Dutch court has extended the detention of the 37-year-old man suspected of opening fire in a tram in the central city of Utrecht, in an attack that killed four passengers.

The suspect, 37-year-old Gokmen Tanis, appeared at a behind-closed-doors hearing Thursday where his detention was extended by 90 days.

Tanis has been charged with murder with terrorist intent and other offenses after he allegedly opened fire in a tram in Utrecht on March 18. Prosecutors have said that Tanis confessed to the shooting and said he acted alone, but they are still investigating his motive.

Prosecutors say he must make his first appearance in open court within three months, when progress in the investigation will be discussed.