Story highlights While we watch one teen's story, a darker tale is unfolding in the background

"Everyone has the power to intervene and get help," Sandy Hook mother says

(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.

"These acts are preventable when you know the signs. Everyone has the power to intervene and get help. These actions can save lives."

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