OK, first of all, just watch this.
This shocking video is the work of the Connecticut nonprofit Sandy Hook Promise, which was set up after the 2012 Newtown massacre.
The PSA, released on Friday, is a part of the gun-violence prevention group's campaign called "Know the Signs."
"People who are at risk of hurting themselves or others often show signs and signals before an act of violence takes place," the Sandy Hook Promise website reads.
"When you don’t know what to look for, it can be easy to miss signs, or dismiss them as unimportant, sometimes with tragic consequences," it says.
In the video, an exchange of messages occurs between this student, Evan, and a classmate.
"I am bored," Evan writes. "Hi bored, nice to meet you," the mystery student responds.
The exchange continues...
And Evan tries desperately to figure out who he's writing to...
When the library closes, Evan thinks he's missed his chance.
But he discovers the mystery girl when he signs her friend's yearbook.
This joyful meeting is interrupted...by a school shooter.
Students scatter in panic...
And then comes this chilling message.
The PSA then goes through each "sign" of the shooter that the viewer missed.
Tons of people have responded to the stunning video. One person said that the PSA gave him chills.
And the shocked emoji face was everywhere.
"Shocking. Frightening. Devastating. True."
But others thought it was far from "true." This person thought the video was proof of "liberals' flippant relationship with facts."
And another called the PSA "ridiculous."
But the response was largely positive, with this Twitter user writing that the PSA "illustrates the problem perfectly."
Remy Smidt is a reporter with BuzzFeed News and is based in New York.
Contact Remy Smidt at firstname.lastname@example.org.
Got a confidential tip? Submit it here.