Authorities had been looking for the man, identified as Korry Martinson, 19, since Friday at about 7:20 p.m., after he allegedly posted the threat on his Facebook page, police said.
The threat was seen by several people, who reported it to the Sedro-Woolley police and the Skagit County Sheriff's Office.
Martinson's mother also was contacted by several callers. When she confronted her son about the posting, he got into an argument with her and took off before police arrived at his home.
Authorities had been looking for him ever since, and the FBI was monitoring the case, said Lt. Lin Tucker of the Sedro-Woolley Police Department.
Martinson finally turned himself in on Sunday afternoon, when a friend brought him to the Sedro-Woolley police station. He was placed under arrest and booked into the Skagit County Jail for investigation felony harassment.
A screenshot of the Facebook threat allegedly posted by Martinson said:
"Okay, so about that shooting of the little kids.
"I would personally like to thank the man who did this. You will (be) looked u pon as a hero in my eyes. You have rid the world of 20 future s------- and w-----.
"I say good riddance, and that we need more people like you. It's the government's fault as to why these things happen.
"If this causes our gun laws to be taken away, to the point as to where I cannot own a gun, I will personally get my sawed off double barreled shotgun and my AK-47 and go shoot up every school within a 100-mile radius of my current location."
Lt. Tucker said the threats shook up many people in the community.
"I've got our superintendent of the Sedro-Woolley School District talking to my boss, wondering, 'Well, should we lock down the schools? ... What should we do for school tomorrow?' I've got the director of security for Skagit Valley College asking the same questions," Tucker told KOMO Newsradio.
Tucker said it's not clear whether Martinson actually owns any guns, and he had no criminal record before making the Facebook threat.
"He's been described as, he likes to stir the pot, get people riled up, his friends said. I don't think he realized how much he was going to stir people up," Tucker said.
"I don't think he understands the can of worms he opened up. You hate to see somebody's life ruined this way, but I also hate to have a bunch of people worrying about sending their kids to school."
Martinson is scheduled to make an initial court appearance Monday in Skagit County Superior Court.