Nearly two weeks after the killing of a 21-year-old man and his infant son, detectives say they now have strong evidence that the victims stumbled upon the home invasion of a well-known drug den and that the father was not the target of a gang hit.

The evidence stems from the emergence of a third witness in the case who was found and interviewed by investigators Monday -- 10 days after the Sept. 14 slayings. Sacramento County Sheriff's Detective Dan Cabral said his statements corroborated the theory that there was nothing in Sean Aquitania's past "that created his demise at all."

Instead, detectives believe Aquitania was killed trying to defend his 7-month-old son, Sean Jr., from the attackers. Aquitania waged two fierce fights with the intruders as the three other men who had been in the house ran away in fear, investigators said Wednesday.

Aquitania, a boxer, eventually was shot multiple times. As the killers left the scene, one of them shot and killed Aquitania's son in the head as the baby sat in his car seat, detectives said.

"I don't think it's ever going to come out that Sean was involved in any criminal activity that led to this, other than associating with some people that should have made different choices," Cabral said.

The slaying of the baby has outraged the community, detectives and even hard-core gang members who have called detectives with tips.

But the case has frustrated investigators, who have probed a number of theories about what happened that day after Aquitania showed up at the house to visit a friend.

Initially, detectives believed Aquitania was in the wrong place at the wrong time and was killed trying to save his baby's life.

Later, authorities said they were probing whether Aquitania's involvement in gangs may have played a role in his death. Aquitania admitted to a sheriff's deputy in 2004 that he was affiliated with the Norteño street gang, according to Detective Brian Meux.

On Monday, Sheriff John McGinness told The Bee "these tragedies are quite possibly a byproduct of bad choices and criminal activity on the part of Sean Sr."

But that same day, detectives located a third witness who was at the home during the shooting and interviewed him extensively about the case, gaining new details and corroborating others.

The result is evidence that Aquitania pulled up to the home just as the robbery of the drug house was to begin, detectives and McGinness told The Bee in a lengthy interview session Wednesday.

According to detectives, Aquitania brought his son to the house on Country Greens Court to "show him off" and to meet with one of the home's occupants, a 21-year-old man who was to be Sean Jr.'s godfather. The godfather was leaving town and "before he left, (Aquitania) wanted to make sure the godfather met the baby," Cabral said.

While detectives do not believe Aquitania was there looking for drugs, "he had to be aware what was going on there, he knew they sold drugs," Cabral said.

As Aquitania pulled up in front of the house shortly before 2 p.m., he was confronted by two gunmen who forced him to the home's front door, detectives said. The two men, with Aquitania in tow, then pushed their way into the house at gunpoint, authorities said.

After a brief struggle with Aquitania, the gunmen "subdued the people in the house," Cabral said. Soon after, Aquitania took on the intruders by himself as the other men fled.

"There was an opportunity that Sean felt he could deal with these guys and he took it and lost," Cabral said.

Meux said Aquitania put up "a fairly violent fight" and that there is "a good possibility (the killers) were injured" during the brawl.

Two of the men who had fled the house later returned to the scene. One found Aquitania inside the home, while the other -- the baby's godfather-to-be -- checked on Sean Jr., detectives said.

"He sees that the baby is crying and he thinks that he's OK," Cabral said. "He pulls the baby out of the car, he sees the wound to his head and he just freaks."

The Bee's Ryan Lillis can be reached at (916) 321-1085 or rlillis@sacbee.com. Bee staff writer Chelsea Phua contributed to this report.