(CNN)Evan finally bumped into the girl he'd been searching for over the entire school year. They had been leaving each other notes on a desk, neither knowing the identity of their mysterious pen pal. But the much-awaited encounter was cut short when a single gunman appeared behind them at the school's gym entrance.
In a chilling new public service announcement, Sandy Hook Promise -- the gun violence prevention group led by families of those killed at Sandy Hook Elementary School -- conveys a powerful message, concealing and then revealing signs that may lead to gun violence.
While we watch Evan's story, a darker tale is unfolding in the background: the evolution of a school shooter. But unless you're watching closely, you'll miss it. And that's kind of the point of the PSA.
"We wanted to create an impactful visual to show that violence is preventable if you know the signs. Many people are unaware of that there are specific signs that people give off that can indicate a violent act is imminent," said Nicole Hockley, co-founder and managing director of Sandy Hook Promise. Her first-grade son, Dylan, died in the Sandy Hook massacre.
"We want this video to inspire hope in those who watch it, to show them that we are not helpless in the face of gun violence and that there is something all of us can do to prevent it," Hockley added.
Stephen Teret, founding director of the Johns Hopkins Center for Gun Policy and Research, said the video is powerful and "extraordinarily well done. It just flipped my stomach at the end.
"I'd like to see the take-home message to be, we need a prevention policy in addition to the students, parents and teachers spotting a student creating havoc.
Teret added, however, that "policy is difficult to make in this area."
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On December 14, 2012, Adam Lanza killed 20 children and six adults at Sandy Hook Elementary School in Newtown, Connecticut, in one of the worst mass shootings in US history. Lanza also killed his mother at their home.
The families of nine of the Sandy Hook victims filed a lawsuit against the gun manufacture, but it was dismissed. On Thursday, the Connecticut Supreme Court said it will hear an appeal from the families.