Toxicology tests came back negative today for drugs and alcohol for the Turlock man who savagely beat his young son to death last month, adding to the mystery of why a father would do such a horrific thing.
Sergio Casian Aguiar, 27, had no drugs or alcohol in his system that might help explain his behavior, said Kristi Ah-You of the Stanislaus County coroner's office.
Aguiar killed his 25-month-old son, also named Sergio, on June 14, the night before Father's Day, on a dark country road 15 miles south of Modesto.
Confronted by a passer-by, Aguiar only stopped kicking the child when a Modesto police officer shot him in the forehead, killing him where he stood.
The boy later was pronounced dead at Emanuel Medical Center in Turlock.
The crime shocked the community with its brutality and generated reaction from around the world.
Investigators and co-workers said Aguiar had no criminal history or signs of mental illness, though witnesses on the night of the attack said Aguiar spoke of "demons" within his son.
Aguiar worked at a FoodMaxx store in Turlock and recently had separated from his wife, Frances Lilian Casian, a kindergarten teacher in Delhi. Casian was in Southern California when the brutal attack unfolded on a dark stretch of West Bradbury Road. She was away for the weekend, authorities said.
Nothing, Casian told detectives, foretold the tragic events.
"Everything is normal, normal, normal, then the incident, then the beating. He just snapped," said sheriff's deputy Royjindar Singh a few days after the incident. "There's no reason why someone would do this. It doesn't make sense."
Aguiar had moved to Turlock legally from Mexico, Singh said, but when he immigrated remains unclear. His only addresses in the United States are listed in Turlock: a trailer he shared with Casian on South First Street, Casian's apartment in the middle of town and her parents' home near Fulkerth Road. Detectives have contacted Aguiar's family in Mexico, Singh said.