The autopsy found water in the 19-year-old’s lungs, ruling out the possibility he was already dead before being pushed into Rome's Tiber river in the early hours of last Friday, Ansa reported, citing sources.
The results come hours after his parents, who flew to the capital on Sunday, met Pope Francis at the Vatican.
The pontiff, who met Solomon's parents at the start of an audience with French pilgrims, assured them of his "compassion and spiritual closeness through prayer", Vatican Spokesman Federico Lombardi said in a note.
The teenager's body was found on Monday after he disappeared in the early hours of Friday following an evening out in the hip Trastevere district near the Tiber with fellow foreign students.
The young sports fan, who reportedly survived a rare form of cancer as a child and hoped to enter politics after university, suffered a wound to the head and his shirt was bloodied. Police suspect he may have hit his head on a rock when he fell.
Investigators believe Solomon, in Rome for a five-week summer course at the John Cabot University, was robbed before his death, forced to hand over his wallet and mobile phone.
He is then thought to have chased after the thieves, following them to the river.
It is there he apparently ran into and brawled with 40-year-old homeless Italian Massimo Galioto, who was arrested on Tuesday for manslaughter.
After witnesses told police they had seen a man being thrown into the water following a drunken altercation, a grainy video emerged, filmed by the owner of a summer festival stall still open on the opposite bank, which showed Galioto and Solomon.
And footage recovered from a security camera in the area captures the moment the Wisconsin youngster fell in the river.
Galioto's companion, with whom he shared a tent on the embankment, told journalists he had not meant to hurt the student.
"They began to argue. They were pushing each other. Massimo pushed him, the boy pushed back, and then he fell in," she said.
Galioto was detained for questioning over the weekend with seven fellow homeless people from the area, but was initially released, before being arrested after he was implicated by the rest of the group.
La Repubblica daily said prosecutor Marcello Monteleone was investigating whether Solomon had been a victim of a criminal gang specialised in robbing tourists before his death.
His bank card was used in the days he was missing by someone who withdrew over €1,500 ($1,675) in Milan.
La Repubblica also carried an interview it had done last year with Galioto, on being a homeless man in Rome, in which he described life in his tent with his companion and their mongrel dogs.
The self-declared former fishmonger said his world fell apart when he caught his wife in bed with another man.
He severed all ties with his family, including his now 17-year old son, and was making what money he could through street juggling, before the arrest warrant fell.