SANTA CLARITA, Calif. — A 16-year-old boy planned the attack that killed two students and wounded three others at a Southern California high school, but investigators were so far unable to find out why he brought a gun to campus and opened fire, authorities said Friday.

After more than 40 interviews and evaluation of evidence, no motive had been established, said Capt. Kent Wegener, of the Los Angeles County Sheriff’s Department’s homicide unit. He said no manifesto, diary or suicide note had been found.

“It still remains a mystery why,” Sheriff Alex Villanueva told a press conference.