The teenage suspect was arrested early Tuesday at his home in Edmonds for investigation of cyber-stalking and harassment.
According to a letter sent to parents from Principal Lisa Hechtman, investigators do not believe the suspect had "active plans or means" to carry out the attack but they are taking the threat very seriously.
Classes at the school were cancelled September 20 after a person claiming to be a Skyline student posted online that he was going to take his father's gun to school and "open fire on the people in the commons in the morning until I am either taken down by our school's police officer, or until I run out of mags."
The threat was quickly circulated on various social media platforms, and several people notified police.
Detectives traced the post to a proxy server in Sweden, which was used to hide the poster's location.
At a news conference on Tuesday, officials said it was information from other students given to the school resource officer at Skyline that eventually helped lead police to the suspect.
Investigators served a search warrant last week at the teen's home and seized three computers and two cell phones, but no guns were found. Officials said evidence from one of the computers taken in the search linked the teen to the threat made against the school.
"That picture, as well as the source of that picture, were found on the suspect's computer," said Sammamish Police Chief Nate Elledge.
Sammamish Police Sgt. Jessica Sullivan said detectives also found evidence indicating that the suspect made a similar threat against Meadowdale High School in Lynnwood last spring.
Elledge said the teen did not seem surprised when police knocked on his door. Skyline students also said they're not surprised by the arrest.
Hechtman said the boy only attended Skyline for a year, but some students have known him for years.
"Well he was a big trouble maker all through middle school," said student Sadie Luther.