AP
Former President Barack Obama called the mass shooting in Las Vegas on Sunday night - the deadliest in modern US history - "another senseless tragedy."
"Michelle & I are praying for the victims in Las Vegas," he tweeted Monday morning. "Our thoughts are with their families & everyone enduring another senseless tragedy."
Beginning in the days following the Sandy Hook Elementary School massacre in 2012, Obama aggressively advocated for more gun-control and safety measures during his time in office, and called his failure to pass significant reforms one of the "greatest frustrations" of his presidency.
He delivered 17 speeches following mass shootings during his eight years in office, which saw 37 mass shootings on American soil.
"This is not normal. We can't let it become normal," Obama said during a speech after a 2015 shooting at a Planned Parenthood clinic in Colorado. "If we truly care about this - if we're going to offer up our thoughts and prayers again, for God knows how many times, with a truly clean conscience - then we have to do something about the easy accessibility of weapons of war on our streets to people who have no business wielding them. Period. Enough is enough."
During a January 2016 speech in which he unveiled a new set of gun regulations, the president broke down in tears while calling on lawmakers to support "common-sense" gun-control measures. He blamed the powerful gun lobby, led by the National Rifle Association, for pressuring lawmakers to oppose even the most basic gun regulations and safety measures, calling the industry "almost entirely unaccountable" in a 2016 op-ed.
"Each time this comes up, we are fed the excuse that common-sense reforms like background checks might not have stopped the last massacre, or the one before that, or the one before that, so why bother trying," he said. "I reject that thinking. We know we can't stop every act of violence, every act of evil in the world. But maybe we could try to stop one act of evil, one act of violence … So the gun lobby may be holding Congress hostage right now, but they cannot hold America hostage. We do not have to accept this carnage as the price of freedom."
The Las Vegas shooter, identified by police as Stephen Paddock, opened fire from the 32nd floor of the Mandalay Bay Resort and Casino, aiming into a tightly packed crowd of many thousands of people attending the Route 91 Harvest country music festival.
Paddock, a 64-year-old from Mesquite, Nevada who appeared to have been armed with at least one automatic weapon, used the elevated vantage point to fire into the crowds across the Las Vegas Strip at the Route 91 Harvest festival, where Jason Aldean was onstage.
Soon after the shooting, which began at 10:08 p.m. PDT, a SWAT team stormed the building, forced their way into his hotel room, and found the gunman already dead. Officers believe he was the only attacker, and that he killed himself before they arrived.
Police discovered at least 10 rifles in his hotel room, which he checked into on Thursday, September 28.
Kieran Corcoran contributed to this report.