"Why leave our children in someone else's hands," Ervin Lupoe wrote in a letter posted on the KABC-TV web site.
The station called police after receiving the fax, and a police dispatch centre also received a call from a man who stated: "I just returned home and my whole family's been shot."
Officers rushed to the home in Wilmington, a small community between the ports of Los Angeles and Long Beach, apparently within minutes of the killings. Officers could still smell the gunshot residue in the air.
Although the fax asserted that Ana Lupoe planning the killings of the whole family, police Lieutenant John Romero said Ervin Lupoe was the suspect. A revolver was found next to his body.
Ana Lupoe's body was found in an upstairs bedroom with the bodies of the couple's twin two-year-old boys.
The bodies of an eight-year-old girl and twin five-year-old girls were found alongside Ervin Lupoe's in another bedroom.
Kaiser Permanente Medical Centre West Los Angeles released a statement confirming both Lupoe and his wife had worked there; both were medical technicians.
"We are deeply saddened to hear of the deaths of the Lupoe family," it said in a statement.
In his letter, Ervin Lupoe claimed he and his wife both had been fired and that she suggested they kill themselves and their children, too. Police described the fax but did not release details.