The prosecutors were treating the case as a potential act of violence committed by a person holding public office, the media reported.
Elysee spokesman Bruno Roger-Petit explained earlier on Thursday that Benalla had asked for authorization to participate in a law enforcement operation, which he received, but then overstepped his authority by intervening physically.
"He was penalized with a 15-day suspension from work and salary suspension, and was relieved of his duties to organize security during the president's trips," Roger-Petit said, as quoted by the France Info broadcaster.
According to the Elysee spokesman, Benalla was warned that any further notices would lead to him being fired.
The investigation has been prompted by media reports saying that Benalla was identified in a May 1 video where he was captured tackling a young man to the ground and punching him repeatedly before fleeing.
The annual May Day demonstration at la Place de la Contrescarpe led to clashes with police and hundreds of arrests. The French newspaper, Le Monde, said Macon’s associate wore a riot police helmet.