The man, held since the attempt to force through a security control at the tower on Saturday night, was examined by a psychiatrist who determined that he lacked discernment, a judicial source said on Tuesday.
The source added that the suspect was sent to a hospital where he had previously been treated.
Investigating judges are looking into possible charges including attempted terrorist murder of public officials and may seek a second psychiatric report, the source added.
The Eiffel Tower was evacuated after midnight on Saturday following the incident, in which the intruder was quickly overcome and arrested without causing any injuries.
Anti-terrorism investigators took over the probe on Sunday. A source close to the inquiry said that under questioning, the suspect had said that he had wanted to attack a soldier and that a member of the Islamic State extremist group had encouraged him to act.
An armed French soldier of the 35th RAP (35e regiment d'artillerie parachutiste), part of Operation Sentinelle, patrolling under at the Eiffel tower in Paris. File photo, July 20, 2016
A knife-wielding man held after allegedly attempting to force his way into the Eiffel Tower in Paris has been sent to a psychiatric hospital.
