Transcript for Sandy Hook: Five years later, victims' families find new purpose
Reporter: It happened again. Last Thursday, gunfire piercing through the sanctity of a classroom at aztec high school in New Mexico. We could hear casings hitting the floor. Scohool shooting -- Another school shooting -- Reporter: A scene too familiar in a country grappling with one mass shooting after another. More than 300 a year on average since 2013, according to the gun violence archive. Nine killed at umpqua community college in Oregon. Four at Marysville high school in Washington state. And of course the one seared in all of our minds, sandy hook elementary school. The mass shooting many hoped would be the last. 26 gunned down. 20 of them 6 and 7-year-old children. Mark Barden's son Daniel was only 7 years old when he was murdered. Mark called him his special little buddy. This is Jackie and Daniel on his last Thanksgiving with us. You just never know. Which of those precious moments. Daniel was an exceptionally sweet, compassionate little soul. He literally used to look out for other people. We used to call him the caretaker of all living things. Reporter: While the million Reese of Daniel are everywhere, the absence he left behind is immeasurable. Another life lost that day, Nicole Hockley's 6-year-old son Dyl Dylan. Dylan was just pure love. He had these gorgeous blue eyes and this amazing smile and this laugh, this giggle that people used to tickle him just to hear him giggle. I see you've got several butterflies. Yes. So being autistic, one of the repetitive movements that Dylan had was to flap. And he would flap his arms up and down whenever he got excited. Which is pretty much all the time. And I asked him once, why do you flap? And he said, because I'm a beautiful butterfly. And at his funeral, I talked about how the theory of a butterfly flapping its wings on one side of the world can cause a hurricane on the other side. Reporter: In their sorrow, Nicole and mark found a new purpose, to prevent school shootings, not through legislation but by thwarting the path of a would-be shooter, starting sandy hook promise. I'm here at the scene of tomorrow's shooting -- Reporter: The foundation uses pas and grassroots efforts training schools to recognize the warning signs and act before it's too late. Because gun regulation, Nicole says, is too controversial. There's so much of a fight that occurs whenever you mention the word gun. We need to reframe this conversation, take the gun out of the discussion, and focus on what do we need to do to keep each other and our kids safe? Reporter: In the immediate aftermath of the shooting, a storm of outrage was felt across the country. Shame on NRA! Reporter: Many called for regulation to curb gun violence. We are killing each other. Reporter: Saying that the loss of so many young delicate lives must lead to change. We're going to have to come together and take meaningful action. Reporter: Momentum seemed to be building toward gun control. First a push to renew the ban on assault weapons. Together we are introducing legislation -- Reporter: Then a bipartisan bill introduced requiring background checks on all commercial gun sales. But five months after the devastation and fury of sandy hook, these efforts were defeated. A minority in the united States senate decided it wasn't worth it. Reporter: And even as president Obama turned to console Nicole and mark, they had already hatched another plan. After every shooting or every suicide, there are signs and signals given off in advance. Ever since the attack at columbine, people have been looking for what they call a profile. Really, there is no such profile. Reporter: Psychologist Peter Langman has studied school shootings extensively and say while every shooter is different, many let their intentions be known. For example, sometimes students will brag to friends about what they're going to do. Another type of leakage is when students try to recruit someone to join them in the attack. He told some of us that his dad kept a gun in his closet, and he always talked about using it on, you know, the people that bullied him. Reporter: Those warning signs front and center in sandy hook promise's provocative new psa, featuring a hypothetical school shooting set in the future. Someone is expected to tell us after the shooting that the shooter has been posting on social media about doing this for weeks. Reporter: This video is approaching 1 million views since its release this week, according to the foundation. It is not a comfortable piece to watch. But you know what? Gun violence isn't comfortable. Our country needs to wake up. Reporter: On the ground, sandy hook promise says they have trained more than 2.5 million students and teachers across the country. Hey, is THA@ an imaginary sketch book? Reporter: At this middle school assembly in Goshen, new York, students are learning a different way to connect with one another. Yeah, why? Reporter: To make sure no one is isolated. There's a fantastic art club on Tuesdays -- Reporter: Born from an unimaginable tragedy, a simple yet remarkable and sometimes even joyous lesson. There's been a number of school shootings we know we've thwarted. Reporter: Bill Sherlock's wife of 31 years, Mary, had been working at sandy hook for nearly two decades and nearing retirement. But when the shooting began, she ran towards the gunfire. If there was something going on, she was going to be in the middle of it. It's not possible to confirm numbers of foiled shootings. But the foundation says their approach has been effective. One example, at the school where bill's daughter now teaches. A number of separate students came to the school administrator's and said, there's two different people we think are at risk of harming themselves. And they were able to get those kids some help. Reporter: Langman says there needs to be more emphasis on prevention and a system of threat assessment. Attention, this is a drill, we need to lock down -- It seems that all schools have lockdown procedures in place. But very few have the preventive threat assessment procedures. Educating your staff and students about those warning signs and what to do when they see them and having a trained team in place within the school to investigate those threats and intervene as necessary. Reporter: Mark and Nicole say each shooting occurring now in a seemingly endless stream feels horribly raw. But until that torrent comes to an end, they will come to fight. I hope that we're not becoming desensitized to unnecessary, preventible murder. Of our fellow countrymen and women and children. So I'm hoping that people will be inspired with the knowledge that it's preventible. I know so many people feel that same sense of hopelessness, that it keeps happening. But then I have to channel that frustration, that anger, but also my love, into this work that I know is going to save lives. You're never going to give up? Never going to give up, why would I? There's still too many people dying, and I know these acts are preventible. For Dylan I will always keep going. Reporter: For "Nightline" I'm Amy robach in Danbury,
This transcript has been automatically generated and may not be 100% accurate.