Fryberg, 15, a freshman at the school and a member of the football team, died as a result of a self-inflicted gunshot wound, law enforcement officials told ABC News.
One girl died and four others were injured in the gunfire.
Rachel Pomeroy, a junior, said she knew the shooter and last spoke with him on Thursday.
"He was fine the day before. He was being sassy, as always, and good," she said.
While Fryberg's motive wasn't immediately known, Pomeroy said he had just come off suspension for a fight.
Robert Fryberg, who identified himself as Jaylen's brother, reacted in real-time on Twitter. First there was dismay about the shooting and then shock when he tweeted that he heard the name Fryberg.
"I just heard some news and I pray to God that it is not true," he wrote.
Two hours later: "I'm gonna miss you little bro," he tweeted. "Only God can judge you."
Jaylen Fryberg is seen smiling in many pictures on his Facebook page, hunting, participating in sports and surrounded by friends.
In many ways, he appeared to be a typical teen, liking video games and counting Adele and 50 Cent among his favorite musical artists.
His Twitter account yields some insight into what he was feeling in the days leading up to the shooting.
The messages posted to his account earlier this week were filled with angst.
On October 20, he tweeted: "Alright. You f***** got me.... That broke me."
The next day, he wrote: "It breaks me... It actually does... I know it seems like I'm sweating it off... But I'm not.. And I never will be able to.."
Later, he added: "I should have listened.... You were right... The whole time you were right..."
Another tweet read: "If I just laid down..."
On Thursday, his final tweet read: "It won't last.... It'll never last."