Medical examiners say "blunt force head trauma" is what killed the 4-month-old son of a Marine stationed at MCAS Miramar.
The cause of that injury, apparently suffered in the family's quarters on the base, is what San Diego homicide detectives are trying to determine.
Andru Bixby died Thursday morning at Children's Hospital, about 14 hours after his 22-year-old father called 911 to report that Andru had stopped breathing. Andru's mother had been watching him and his 18-month-old brother before leaving for school when the father came home.
That background presents a delicate and complex task for investigators, according to a local criminal defense lawyer not connected to the case.
"The police have to come up with probable cause for an arrest, but they want more than that," legal expert and attorney Gretchen von Helms said in an interview Monday. "They want to make sure that they have proof beyond a reasonable doubt or that they can try to reach that standard for a jury, for a conviction in this case."
Von Helms said that detectives are likely to probe into the 18-month-old boy's health and the mother's relationship with the father -- and may encounter legal counsel for each.
"Obviously, [the mother and father] have different interests," von Helms noted, "because it could have been either one of them -- or neither of them."
Homicide detectives have not responded to requests made on Monday for more information and interviews. Military authorities also are looking into the case.
In the meantime, the child's death has become a source of mourning and cynical speculation on the Internet.
"Rest, little Andru," said one Web posting. "When you wake up, you'll have wings."
"This will end with the mom and dad pointing the fingers at each other," reads another posting.
And there was this: "Sadly, there's no exam you have to pass to be a parent."