A spokesman for the French prosecutor's office confirmed Sunday that an antiterrorist investigation was opened after an interrogation of the suspect, a young Frenchman of Mauritanian origin who, according to preliminary data, professes Islamic extremism.
The man is now under investigation for involvement in a terrorist enterprise and an attempted terrorism-related attack on security forces.
The man wanted to attack a soldier after encouragement from Islamic State, according to a source close to the investigation.
Armed soldiers who are permanently stationed at the Eiffel Tower surrounded him and ordered him to drop his weapon, which he did without attacking anyone, one security source said, asking not to be named.
But the suspect, a French national born in Mauritania in 1998, is thought to have psychiatric problems.
19 countries officially apply to host the EMA; 8 chase EBA
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The tower, which lets visitors enter until midnight, was evacuated an hour later in what a police source said was a standard precaution.
France has been under a state of emergency since November 2015 attacks in Paris that left 130 people dead.
Since the first wave of terror attacks in January 2015, 239 people have been killed.
When arrested on Saturday night, the man was reportedly wearing the colors of the Paris Saint-Germain soccer team.
A screen near the bottom of the tower displayed a message welcoming Brazilian star Neymar to the club.