A horrifying act of violence that took place at a night club in Orlando, Fla. Sunday morning (June 12) is being called the deadliest mass shooting incident in U. S. history.

50 people were killed and at least 53 more were injured in the attack, which took place around 2AM at a gay night club called Pulse. According to CNN,  Omar Mateen of Ft. Pierce, Fla., opened fire inside the nightclub early Sunday, then took hostages in a several-hour standoff with authorities. He was killed after police broke down the door of the building around 5AM with an armored vehicle.

"It appears he was organized and well-prepared," Orlando Police Chief John Mina said early Sunday. Police say Mateen had an assault-type weapon, a handgun and "some type of [other] device on him."

Authorities say a full investigation is ongoing. Orlando police consider the shootings an act of domestic terror, according to CNN. The FBI is involved is involved in the investigation, and they "have suggestions the individual has leanings towards [Islamic terrorism], but right now we can't say definitely," says Ron Hopper, assistant special agent in charge of the FBI's Orlando bureau. Authorities stress that they are still investigating all possible motives, including the possibility that the shootings were a hate crime directed at the gay community.

Reaction online has been intense, with the hashtags #PrayFor Orlando and #PrayForPulse trending online. A number of country stars are among those who have reacted to the horrifying attacks.