But even more incomprehensible to some was the story that emerged after the bodies were found Tuesday: A father who, after he and his wife were fired from their jobs, killed all six family members before turning the gun on himself.
In a letter faxed to Los Angeles television station KABC before his suicide, Ervin Antonio Lupoe blamed his former employer for the deaths, detailing his grievance against Kaiser Permanente's West Los Angeles Medical Center, where he and his wife Ana had worked as technicians.
Lupoe claimed the couple was being investigated for "misrepresentation of our employment to an outside agency for the benefit to ourselves's [sic], childcare." He said the initial interview was held on December 19, and when he reported for work on December 23, "I was told by my administrator ... that 'You should not even have bothered to come to work today. You should have blown your brains out.'"
Kaiser Permanente said in a statement Tuesday night that while the company is "saddened by the despair in Mr. Lupoe's letter faxed to the media ... we are confident that no one told him to take his own life or the lives of his family."
The Lupoes' employment was terminated over a week ago "after an internal investigation," the company said.
Lupoe wrote in the fax, "after a horrendous ordeal my wife felt it better to end our lives and why leave our children in someone else's hands ... we have no job and 5 children under 8 years with no place to go. So here we are."
Among those struggling to comprehend the news was Lupoe's 83-year-old grandmother, Josephine Lupoe of Atlanta, Georgia. She sobbed as she told CNN, "I just can't believe it."
She said Lupoe was born in Atlanta, but moved to Detroit, Michigan, with his parents as a child before moving to California. She recalled visiting the family when they lived in San Jose, California, "but that was years ago," she said. "I hadn't been to visit them since he got married and moved."
She said she last heard from him when he called her to say they were having a second set of twins.
"Every time I called, he was at work," Josephine Lupoe said. "He worked a lot, and even when I talked with him, he would be at work." But she said she had no indication of problems within the family.
She said she had spoken with Lupoe's mother a couple of days ago, and they discussed his sending pictures of the boys. "And then I hear this," she said tearfully.
Lupoe's mother was on the way to California on Wednesday, she said, but Josephine Lupoe said she is unable to travel.
Authorities said Lupoe and the three girls, identified by the Los Angeles Times as 8-year-old Brittney and twin 5-year-olds Jaszmin and Jasseley, were found in one upstairs room. Ana Lupoe and the boys -- twin 2-year-olds Benjamin and Christian, according to the newspaper -- were in another.
Police believe Lupoe also called 911, about the same time KABC was notifying police they had been contacted by a person who was threatening suicide. In the 911 call, police said, Lupoe reported returning home and finding his family dead.
Lupoe's co-workers told the Times they remembered the Lupoes as cheerful, good workers and caring parents.
Ana Lupoe was "always talking about the kids," said co-worker Hamlet Narvaez.
Cherise Pounders-Caver, principal of the children's school, Crescent Heights Elementary, said Lupoe showed up to check the three older children out of school about two weeks ago and told her the family was moving to Kansas, the Times reported.
The deaths sent shock waves across the city and beyond.
"No matter how desperate you are, no matter how frustrated you are, to think this was the only answer -- to take your whole family with you in death -- is just too much to understand," said City Councilwoman Janice Hahn, who represents the city's Wilmington neighborhood where the slayings took place.
"It's sad that this happens anywhere, you know?" neighbor Jose Rodriguez told KABC. "You see it on the news but you never really become accustomed to it.
"I have kids, too, and grandkids," he said. "It hits home."