Buonanotte charged with manslaughter
Buonanotte, 22, was driving during a storm when his car overturned and crashed into a tree in his home province of Santa Fe on December 26.
He suffered injuries to a shoulder and lungs, but his three childhood friends were killed.
"Buonanotte is charged with triple manslaughter and eventually this is likely to go to court to determine whether he is guilty," Carlos Colimedaglia, a prosecutor in Junin, Buenos Aires province, told Reuters.
"Speed and weather conditions at the time of the accident made him lose control of the car," Colimedaglia said, adding that Buonanotte had tested negative in an alcohol test.
Buonanotte has said he cannot remember how the accident happened, but that he was driving at a prudent speed, daily La Nacion reported. He testified for 40 minutes on Monday.
Buonanotte resumed playing in April, almost four months after the fatal crash.
His legal assistant told a local radio station on Monday the case against the player "lacked sufficient legal grounds."
"In all traffic accidents the prosecutor tries to determine whether the actions of anyone involved have contributed to the outcome," the assistant, Dario Del Ciervo, said.