A gunman walked into a gay nightclub in Orlando early Sunday morning, in the middle of Pride Month, and began shooting those partying inside with a handgun and an assault rifle. By the time police stormed the building three hours later, Pulse had become the scene of the deadliest mass shooting in American history.

At least 50 people are dead. At least 53 more people were injured; some of the injuries are severe, and the death toll is reportedly expected to rise.

For comparison, the second worst mass shooting in the U.S happened at Virginia Tech in 2007. Thirty-two people were murdered. In 2012, 20 first graders and six staff members were killed at Sandy Hook Elementary.

Both the state of Florida and Orlando have declared a state of emergency.

Early this morning, the nightclub, Pulse Orlando, posted on its Facebook page, "As soon as we have any information we will update everyone. Please keep everyone in your prayers as we work through this tragic event. Thank you for your thoughts and love." Hours earlier, the same Facebook page had posted, "Everyone get out of pulse and keep running."

Pulse, which calls itself "Orlando’s Latin Hotspot," was opened in 2004 by the sister of a man who died from AIDS. "It was important to create an atmosphere that embraced the gay lifestyle with décor that would make John proud," Barbara Poma says on the nightclub’s website. "Most importantly, [we] coined the name Pulse for John’s heartbeat — as a club that is John’s inspiration, where he is kept alive in the eyes of his friends and family."

On Saturday, the club was holding its regular "Upscale Latin Saturdays" party; according to Orlando police chief John Mina, there were about 300 people inside when shots started going off at 2 a.m. "At first, when I heard the shots I thought it was part of the music," one person told the Orlando Sentinel, "but then we realized it was really happening. I started running and saw someone bleeding from the arm and I was like wow this is really happening and I kept running." A mother of someone inside the club told the Sentinel that her son called around 2:30, screaming, "Mom, there's dead bodies."

Police officers stationed outside Pulse, worried about potential hostages inside, finally entered the building around 5 a.m. with an armored vehicle, per the New York Times. A few officers exchanged gunfire with the shooter, who was killed.

Sunday morning, after the attack, the people of Orlando immediately mobilized to do whatever they could to help. The lines outside blood banks remain long as of this writing, and the local LGBT community center has counselors ready to talk via hotline or in person. Rob Domenico, affiliated with the Center Orlando, told NBC News, "This is a time, more than ever, that we need to put the unity back in community and to stand up to show love and compassion to those going through this horrible travesty ... Step out of your comfort zone. If you've never volunteered, now is the time you're needed."

Equality Florida has released a statement, according to the Los Angeles Times, that said, "Gay clubs hold a significant place in LGBTQ history. They were often the only safe gathering place and this horrific act strikes directly at our sense of safety … We make no assumptions on motive. We will await the details in tears of sadness and anger. We stand in solidarity and keep our thoughts on all whose lives have been lost or altered forever in this tragedy."

Right now, the investigation is focused on the shooter, identified as 29-year-old American citizen Omar Mateen. He lived in Port St. Lucie, Florida, 124 miles away from the club. The gunman’s father, Mir Seddique, reached by NBC News, said in a statement that "We’re apologizing for the whole incident. We weren’t aware of any action he is taking. We are in shock like the whole country. This had nothing to do with religion." Seddique added that he remembered his son being furious after watching two men kiss in Miami a few months ago. The FBI said at a news conference on Sunday that it was investigating whether the attacker was influenced by ISIS; the Council on American-Islamic Relations’s chapter in Florida is pushing the local Muslim community to take part in blood drives, and called the mass shooting "monstrous."

A year ago this week, nine black people attending Bible study were killed at Emanuel AME Church in Charleston. The Associated Press published a story checking back in with the city only hours after the shooting in Orlando. "I think a lot of things happened out of the immediate emotions of how horrific the killings were," Dot Scott, president of the local NAACP chapter, told the AP. "That's the human side of folks and the politeness, particularly of Charleston, that we just had to do something. But then when reality checks us — the question is what is that going to cost us in terms of changing the way we think and do things?"

Update [6/12/16, 6:30 p.m.]: In the middle of the nightclub attack, shooter Omar Mateen reportedly called 911 and talked about ISIS and the Boston Marathon bombers. NBC News reports that the FBI talked to the shooter in 2013 and 2014 regarding alleged connections to terrorists; the investigations were closed in both cases. The ATF says that the gunman legally purchased his weapons in Florida in the past week. Mateen’s ex-wife has told the Washington Post that her ex-husband wasn’t a “stable person. He beat me. He would just come home and start beating me up because the laundry wasn’t finished or something like that.”

In an address this afternoon, President Obama said, “Although it’s still early in the investigation, we know enough to say that this was an act of terror and an act of hate.” The Wall Street Journal reports that the FBI is “probing the shooting as a terror attack inspired by a foreign terror organization,” but “this doesn’t rule out that it could also be investigated as a hate crime.”

Meanwhile, a man was arrested in Santa Monica with guns and possible explosives in his car. He allegedly told one officer that he planned on harming those at the Los Angeles Pride parade, per the Los Angeles Times. The L.A. Pride Festival went on as planned, with many people at the event bearing signs mentioning Orlando.

Extra police officers were stationed at Pride parades in D.C. and Philadelphia on Sunday.