Many recent mass shootings share a troubling connection: The accused killers have left a blueprint for their alleged actions on social media, like the attack on a Pittsburgh synagogue last year in which 11 people were killed, and the massacre in Christchurch, New Zealand in which 51 were killed.
The alleged gunman inposted his anti-immigrant manifesto to the online message board 8chan minutes before the shooting. Cloudflare, the network provider for 8chan, has since announced it is dropping the site. But the El Paso suspect was not the first to use 8chan; other mass shooting suspects have done the same.
"You have would-be recruiters and provokers who are using these platforms to get to people on a personal level," said Nate Snyder, a former counter-terrorism official in the Department of Homeland Security.
The online posts are out in the open, but difficult for law enforcement to spot unless they get a tip before an attack happens.
Inside a conference room in an unmarked building, the FBI has been working to prevent future attacks. It's where analysts comb through dozens of recent active shooting cases.
Agent Andre Simons says they have been looking for patterns.
Correspondent Jeff Pegues asked, "It's not really accurate to say that someone just snapped?"
"Not at all," Agent Simons replied. "In fact, what we found is that all of the shooters in our study certainly planned and prepared. Many shooters spent almost two years planning their attack."
FBI studies have found that only 25 percent of active shooters had ever been diagnosed with mental illness, and that shooters aren't typically loners. Twenty-seven percent had significant online interaction.
When asked what the motivating factors are for shooters, Simons said, "Usually it's a desire for some omnipotent control, even if it's just momentary. There is also a degree of desire for infamy and notoriety."
The FBI tells CBS News they receive three to four referrals a week of people who've landed on law enforcement's radar. But in the majority of cases, people who see signs of trouble before mass shootings often don't call police.
In a rare statement from the FBI, the agency said the attack in El Paso "underscores the continued threat posed by domestic violent extremists and perpetrators of hate crimes."