Fryberg, a freshman at the school, is cousins with two of the wounded students, according to witnesses and the boy's own grandfather.
"All three of them are cousins, and they live right close to each other," Don Hatch told CNN affiliate KOMO.
Police found Fryberg dead in the school from a self-inflicted gunshot wound. A female student also died in the shooting.
Four wounded students were hospitalized in the intensive care unit with "very serious" injuries. They are all under the age of 18.
According to witness Jordan Luton, Fryberg approached a table of students, "came up from behind ... and fired about six bullets into the backs of them." Luton told CNN, "They were his friends, so it wasn't just random."