Monday's 5 things: The deadliest mass shooting in U.S. history

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    Orlando gunman was on FBI's radar

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(CNN)America suffered the deadliest mass shooting in its history early yesterday. As we grapple with the tragedy, we're ditching the levity in this morning's 5 Things. Here's what you need to know about the shooting, the aftermath and, most importantly, how you can help.

1. The shooting

    It began just after 2 a.m. -- closing time at Pulse, which is more than a gay nightclub in Orlando; it's a sanctuary for many. Omar Mateen, armed with an assault rifle and a pistol, opened fire on about 300 people, killing at least 50 and wounding 53 others, the deadliest mass shooting in U.S. history and the worst terror attack on U.S. soil since 9/11. After a three-hour standoff -- as clubgoers desperately called and messaged loved ones for help -- police stormed the club with an armored vehicle and killed Mateen. Here's how the shooting unfolded.

    2. The gunman

    Omar Mir Seddique Mateen was born in New York to Afghan parents. He worked as a security officer in Fort Pierce, Florida. The FBI interviewed him twice in some terror-related cases, but nothing came of it. He unnerved co-workers with talk of possible terrorist ties; his ex-wife said he was abusive and mentally ill. In a 911 call shortly after his rampage, he pledged allegiance to ISIS. What led him to attack Pulse is still unclear, but his father said he was outraged after seeing two men kiss on the street.

    3. The victims and survivors

    When the shooting started, many inside thought the pops of gunfire were just part of the music. As it became clear that something was seriously wrong, those trapped inside did whatever they could to survive - some covering themselves under dead bodies. As first responders made their way through the dead bodies, they called out, "If you are alive, raise your hand." Around them, the incessant chirping of victims' unanswered cell phones comprised a soundtrack of disaster. Here's what we know about those who were slain.

    4. The reaction

    Like he did 14 other times during his presidency, Barack Obama somberly addressed the nation yet again about a mass shooting. And yet again, the familiar fault lines were exposed: the Democrats demanding gun control; Republicans railing about radical Islamism. From the Queen to the Pope to everyday people, a flood of sympathy swept through the world. The Tonys went on as scheduled, but it too honored the victims.

    5. How to help

    You can give blood: Orlando's local blood bank met its needs yesterday, but will need more in the coming days, especially the rarer O negative, O positive and AB plasma blood types. You can also give money: Equality Florida, a nonprofit LGBT civil rights organization, set up a GoFundMe account for the victims and their families. It raised $500,000 in just a few hours.