Chris Cuomo welcomed David Cullen, author of "Columbine" and someone who's become something of an expert on the topic of school shootings.

"The police aren't the answer here. What are the answers?" Cuomo said. "The question: What do we do to stop the school shootings? Is there an answer?"

"There's a couple things we can do," Cullen said.

"I was in the infantry. Seeing that rifle, it looks just like the M-16 we used. An M-16 is to kill people. We were trained to kill people with it. They're assault weapons. They're military weapons. They should be limited to the military."

"Would it matter?" Cuomo said.

"It would totally matter. I've been doing the last couple weeks -- Russian TV," Cullen said.

"It's not reported much here. They've had a string of school stabbings across Russia. The country is in a panic about it because they're pretty horrific. The last two, there were 12 to 15 people seriously injured in each one. I think there's been five of them over the last couple months. Totality fatalities, zero. Nobody dies. It's almost impossible to get a weapon there. This is not a theoretical discussion. We've had this in other countries, too. We have a situation where the same thing is going on, but they don't have access to guns and nobody dies."

Cuomo asked what can be done without touching the gun issue. "Is there any way to reduce the risk without touching what our politicians seem to feel will kill them if they touch it?" he said.

"Yes, there are a couple things we can do," Cullen said.

"I hate to say I'm glad in these situations, but I'm glad this person is identified as extremely depressed right away, so we can have that conversation. Most people don't realize that's the main cohort we're talking about. The Secret Service reports 78% of the school shooters were either -- had either attempted suicide or talked about doing it. Nearly all of them were extremely suicidally depressed. That is the main factor, the main commonality. We should be screening for depression.

"This person had been identified, but most have not. Most teen depression, it usually manifests in adolescents because when your brain starts changing -- it's simple to do. I don't know if you can show it on camera, a one-page questionnaire. It takes one minute to fill out. basic questions like, 'in the last two weeks how often have you had trouble sleeping,' feel depressed and so forth. A great screening tool. It costs nothing. If they pass this out twice a year in homeroom to every kid and score it, we can identify kids and get them help. You're not going to solve every --"

"You'd be doing something, something that can, have to deal with resources. At least it would be a start," Cuomo said.

"A huge start," Cullen replied..