"It's hard to imagine the loss from the shooting in Las Vegas. It's hard to imagine why we don't make it much harder for anyone to do this," Zuckerberg writes on Facebook.
At least 58 people are dead and more than 500 have been injured in the deadliest shooting in U.S. history, after a gunman opened fire at the Route 91 Harvest country music festival across the road from the Mandalay Bay hotel in Las Vegas on Sunday night.
There have been hundreds of mass shootings in the United States already in 2017, and in the wake of especially large and public mass shootings, there is often renewed anger over the gun laws in the United States. Zuckerberg does not explicitly mention a need to tighten access to guns in the U.S.
Zuckerberg says that Facebook has activated its Safety Check feature, a crisis response tool allowing people to check in identifying themselves as safe and for others to check in on friends and loved ones who may have been in the area at the time of the tragedy.
Earlier Monday morning, Zuckerberg colleague and Facebook COO Sheryl Sandberg posted a response to the massacre on her personal Facebook page.
"Thoughts with all affected by the terrible attack in #LasVegas. Keep safe & look out for one another," Branson says.