Five of the Twelve Deadliest Gun Massacres in U.S. History Took Place During Obama's First Term John Cook · 12/14/12 04:00PM Barack Obama was sworn in as America's 44th president 1,425 days ago. Since then, five of the 12 deadliest mass shootings in our history have happened. Here they are, as well as other lesser mass gun-killings that happened under his watch.