School security is an estimated $3 billion industry, including things like training, armed officers, and video cameras. But there’s little evidence on what’s actually effective.

Because gun control measures haven’t gone far, schools are investing in security, says The 74’s Mark Keierleber. Those measures come with significant costs.

In the aftermath of the Sandy Hook shooting, Marketplace looked into background checks and gun purchases. We’re revisiting some of that reporting today.