[from that article]

"The rampage allegedly started over an undetermined family dispute. Public records show that Zawahri's parents were married in 1985 and moved into the house they purchased on Yorkshire Avenue in 1996, but the mother, Randa Abdou,^[35] left the home and moved to an apartment with the two boys in 1998. She sought a restraining order against the father a short time later, but the case was dismissed when the mother failed to appear in court. Subsequently, the elder son lived with the father at the residence on Yorkshire Avenue, while Zawahri lived in an apartment in Mar Vista, Los Angeles with his mother. Though there is no record that the couple divorced, by 2013 they had been living separately for years.^[32] Randa Abdou was out of the country visiting relatives at the time of the shooting, but returned during the following weekend and was assisting authorities in the investigation.^[11][35]

Zawahri prepared a three-page handwritten note that was found on his body. In it, he expressed remorse for killing his father and brother, but did not give a motive. He said goodbye to friends and expressed hope that his mother would be taken care of and receive recompense from his father's estate. Investigators believe that mental illness played a role in the killings, but no details were given. Searching his home, police found replica weapons and illegal zip guns. It was also learned that the California Department of Justice advised Zawahri in an October 2011 letter that he was ineligible to purchase a firearm.^[38] The incident occurred the day before Zawahri's 24th birthday."