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The commenters said they would be at a specific place, waiting to kill the teenager with a gun.
The concerned mother called her son's school, West High School in Utah, to share the comments with administrators.
They agreed; the threats were not a joke.
"They were threatening his life on Facebook. Pretty credible threats. And there were also pictures on Instagram involving a weapon," police Detective Greg Wilking told KUTV.
Police arrested two teenagers on Friday afternoon, using the information boys had posted on social media to find them.
The boys were picked up down the street from West High School -- right where they had said they would be, police said. In the suspects' car, police said they found a handgun, a loaded magazine, marijuana and a large bong.
"Mom obviously was monitoring social media and did the right thing by calling us," Wilking said. "She saw something on there that concerned her and took it seriously, and that's what parents need to be doing; paying attention to the social media that their children are involved with."