No easy answer in how to stop mass shootings, that's the most troubling thing of all: Varney

Oh no, not again. Isn't that what we all felt? You get the news alert. The grim details emerge, and you feel that shock. Why is this happening?

His name is Nikolaus Cruz. He's 19. He's long been considered troubled.

His adoptive parents had died. He had moved in with another family, but was not happy, and late last year, he had moved to a mobile home with another student from Marjory Stoneman Douglas High School. That was the school that expelled him for disciplinary reasons. And that was the school he returned to yesterday to kill 17 people.

There were plenty of warning signs. At school, he was not allowed to carry a backpack. He went to a mental health clinic for treatment, but stopped going. Other students say he liked to kill animals—that’s a classic warning sign. And on social media, his posts are described as "disturbing." He was obsessed with guns.

He used a rifle for his killing spree. It was purchased legally. He went back to that high school, walked inside, started shooting, evidently pulled the fire alarm so kids poured out of their classrooms, and he kept shooting. He escaped, briefly, as other students ran outside. Ninety minutes later, he was in custody.

There are stories of heroism—we are told a football coach died, shielding other students and that endless video loop that we have seen so many times—children, running to safety, hands in the air.

Why did a clearly troubled youngster have access to a gun? If there were so many warning signs, why wasn't he stopped? Can you pass a background check with a history of mental instability? What do you do with troubled kids whose parents have guns? Why is it happening so often in our schools?

This is a very difficult time. It keeps on happening. Surely, every one of us has asked: How do we stop this happening again? And we have no answer, certainly no easy answer. And that is the most troubling thing of all.