Quiroz deployed to Iraq just before his son was born, but he was called back when his wife died in childbirth, and was left to take care of their two children.
Three months later, prosecutors say Quiroz killed the little boy, and told a couple different stories to cover up his role.
The difference between murder and manslaughter lies somewhere inside this West Central Fresno living room, and in the stories a father told about how his infant son died.
Prosecutors say 3-month-old Roman Quiroz died from blunt force trauma to his head -- taking a blow coroners compared to what would happen if he was thrown against a wall. But in his call to 911, /*Robert Quiroz*/ acted like he had no clue why his son stopped breathing.
"Did you see what happened?" a 911 operator asked him.
"No," Quiroz said. "I just came out here and he's not moving or anything."
Firefighters and paramedics arrived at the house a few minutes later.
They say Quiroz showed no sense of urgency to get help for Roman.
"There wasn't any behavior being exhibited by him saying, you know, 'This is the place. This is the place. Come in'," said Fresno Fire Capt. Michael Mollica, who came with the first responders.
Roman's aunts on his mother's side say Robert Quiroz told them two different stories about the damage to his son's head.
A judge wouldn't allow us to show their faces as they testified Tuesday, but one of the women said Quiroz told her he bounced the boy's head off the armchair of his couch. The other got another explanation.
"He said whenever he forcefully took (Roman) off the couch to immediately give him CPR, so his head did kind of hit the floor pretty hard," Anna Childress testified.
Quiroz's attorney says his client admits he deserves the blame for his son's death. But he says it was an accident; Quiroz may have been negligent, and guilty of manslaughter, but he's no murderer.
"He's really a good person that did something unfortunate and tragic and he's probably going to have some responsibility in that regard," said Michael Idiart.
The murder trial is going to last a couple weeks. Quiroz is expected to testify later in the case, probably sometime next week.