As Latin music blared inside Pulse, one of Orlando’s biggest nightclubs on June 12, 2016, a gunman forced his way inside and opened fire on the predominantly gay crowd. In the end, 49 people were dead and dozens more injured, in what was, at the time, the deadliest mass shooting in modern U.S. history.

When the gunman, 29-year-old Omar Mateen of Fort Pierce, Florida entered the club with an AR-15-type assault rifle and a handgun, the nearly 300 people inside were winding down their Latin-themed night of dancing. When the first shots rang out, many described not noticing, thinking the bangs were part of the songs, until people started to fall the floor and others ran in terror. Some hid in the bathrooms.

“I heard 20, 40, 50 shots,” Jon Alamo told BBC. “The music stopped.”

At 02:09, the nightclub posted on its Facebook page: “Everyone get out of pulse and keep running.”

As Mateen moved through the nightclub, he exchanged fire with the club’s security guard and, as more officers arrived on the scene, shots continued to be exchanged. Mateen then escaped to the bathroom, where he took hostages and told the police he had explosives he was ready to detonate.

While the gunman was in the bathroom, police evacuated those still on the club’s dance floor. Many tweeted or texted for help from the inside, including people trapped in the bathroom who hid in the stalls trying not to be seen. Others played dead. During the attack, Mateen called 911 to pledge allegiance to ISIS.

At the same time, officers secured the building and prepared to enter the bathroom using explosives on the outside wall of the building. At about 5 AM, the police stormed through their exploded hole, then shot and killed Mateen.

At the time of the shooting, it was unclear if this was an act of terrorism or a hate crime. While Mateen’s family said that he had shown anger towards two gay men kissing the week before the attack, evidence discovered in the years after the attack shows that this may have been a planned act of terrorism and may have had a different target—a Disney complex—before Mateen got spooked by police.

Mateen had been interviewed by FBI officers twice in 2013, after making comments to coworkers about his connections to ISIS. He was questioned again in 2014 about a potential connection to Moner Mohammad Abu-Salha, an American suicide bomber who had attacked in Syria.

Seven months after the attack, Noor Salman, the Omar Mateen’s wife, was charged with obstruction of justice for making contradictory statements to the FBI, and aiding and abetting for allegedly ignoring her husband’s connections to ISIS. The FBI believed she may have known of his plan.

In March of 2018, she was found not guilty.