Las Vegas shooter Stephen Paddock bought 33 guns within the last year, but that didn't raise any red flags. Neither did the mountains of ammunition he was stockpiling, or the bump stocks found in his hotel room that allow semi-automatic rifles to mimic fully automatic weapons.
Some recent mass shooters in the U.S. have amassed arsenals by exploiting limited weapons laws. Others have taken advantage of blind spots in background checks and obtained guns while undergoing mental health treatment. Others got their hands on weapons that were initially purchased legally.
A look at some recent mass shootings and how the attackers obtained their guns: