The gunman, identified as Jaylen Fryberg, left a series of tortured posts on Twitter in the months leading up to his attack in Washington state, hinting that a failed romance may have been the catalyst for the shooting that left one dead and three critically wounded.
The rampage ended when he took his own life.
Social media posts painted a portrait of a teenager well used to handling firearms, with one image on Instagram showing him brandishing a hunting rifle.
Tweets have suggested a break-up may have been behind the Washington school shooting. (AAP)
"Probably the best BirthDay present ever! I just love my parents!!!!," he posted in a message accompanying the photo of him proudly holding the gun.
In his final post on Twitter on Thursday, Fryberg stated ominously: "It won't last...It'll never last...."
Less than 24 hours later the teenager was lying dead in Marysville-Pilchuck High School from a self-inflicted gunshot wound after opening fire in a crowded cafeteria.
Thursday's Twitter posting was the last in a flurry which began on Monday with the cryptic: "Alright. You [f***ing] got me ... that broke me."
A day later he added: "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." The post was followed by a series of anguished emoticons.
Later on he added: "I should have listened...You were right...The whole time you were right..." A final post on Tuesday added: "If I just laid down."
Two months earlier, on August 20, Fryberg's fury had flared again on Twitter.
"Your gonna piss me off...And then some shits gonna go down and I don't think you'll like it," he raged, following swiftly with "Your not gonna like what happens next."
Later the same day he added: "I hate that I can't live without you."
A month later, Fryberg appeared to be railing against a broken friendship. "Did you forget she was my girlfriend?" he wrote on September 18.
"Dude. She tells me everything. And now. I... hate you! Your no longer my brother.
Social media captures family's learning and grief
Fryberg's elder cousin Robert took to Twitter on Friday to tweet his grief.
"I don't wanna believe it," he wrote. "I'm gonna miss you little bro. Only god can judge you."
In an eery timeline of the unfolding events, Robert's twitter feed reveals that he was concerned after learning of the shooting but was apparently oblivious that Jaylen was at the center of the bloodshed.
Then later he wrote, "I hope no one got shot though! Stay safe!", which was soon followed by:
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