How do you make a portrait of a child who isn't there? Photographer Lou Bopp found a way, but it wasn't easy.

In early 2018, I was deplaning after an 18-hour flight when Steve Hartman called. He had an idea: to photograph the still-intact bedrooms of kids who had been killed in school shootings. 

It's a headful. And six years later, I still don't have an "elevator pitch" for the project — but then, I don't often talk about this project. It is by far the most difficult I have ever worked on.

When Steve, my friend of about 25 years, asked me if I would like to be involved, I said yes without hesitation — even though I didn't think we would get any families to agree. There is no way that I would have said no to partnering with him on this.