Neighbor Merita Callaway said friends of the children found the body of Phillip Marshall, 54, when they went to the family's Murphrys-area home Saturday to check on Alex and Macaila Marshall after not having heard from the teens for several days, The (Stockton) Record reported.
The Calaveras County Sheriff's Office said Alex, 17, and Macaila, 14, were also found dead in the home. Each had been shot once in the head with a handgun, as was the family's dog, the newspaper said.
The motive for the shootings had not been determined.
Sean Marshall, the estranged wife of Phillip Marshall and mother of the children, was returning from Turkey, where she had been traveling at the time of the shootings, The Record said.
The children were students at Bret Harte Union High School in Angels Camp.
The newspaper said Phillip Marshall, a former pilot for Eastern Airlines and United, was an author whose published works included the 2003 novel "Lakefront Airport" and "The Big Bamboozle: 9/11 and the War on Terror," a 2012 book in which he theorized it wasn't al-Qaida but U.S. and Saudi government officials who orchestrated the Sept. 11, 2001, terror attacks on the United States.