President Obama ordered U.S. flags to fly at half-staff on public buildings and grounds in memory of the 50 killed at an Orlando, Fla., nightclub early Sunday morning.
Obama called the shooting that also left 53 injured an act of terror and vowed to dedicate the federal government's full resources to the investigation.
"This was an act of terror and an act of hatred, and as Americans, we are unified in grief and outrage and in resolve to defend our people," Obama said from the press briefing room of the White House. "[The shooter] was a person filled with hatred, and in the coming days, we will uncover why and how this happened."
Fifty people were killed and 53 more wounded inside the crowded gay nightclub Pulse. Omar Mateen, a U.S. citizen from Florida, is suspected in the shooting. He was killed at the scene.
Orlando Police Chief John Mina told reporters the suspect had an assault-style rifle and a handgun.
The casualty figures at Pulse make it the worst mass shooting incident in U.S. history. It also marks Obama's 20th post-shooting address since becoming president.