From Post-Gazette:

Under the keen eye of a trained instructor, who was also Jewish, David was surprised at how comfortable he was handling a gun. As someone who grew up in a community in which owning guns was unthinkable, he was surprised at how many of his friends also owned guns.

“Growing up, the mere thought of owning a gun or handling a gun was taboo. However, once I started having conversations with people after Tree of Life, I found the reality is everybody might be somebody who would buy a gun. It just depends on what it takes to get one,” he explained.