Orlando first responders had to make their way through a 'cacophony of ringing unanswered cellphones'

As authorities piece together the details of Sunday's mass shooting in Orlando, a horrifying picture is starting to emerge of the three-hour attack and hostage situation that occurred inside the Pulse nightclub.

Pulse was nearing closing time when Omar Mateen reportedly entered with an assault rifle around 2 a.m. The music was blaring and the estimated 300 people were still enjoying the night in the darkness of the club when the first shots rang out. Initially, the staccato of Mateen's discharging semiautomatic weapon blended with the beat of the music, survivors said.

"No one put two and two together until the fifth and sixth [shot]," one survivor told CNN. "Between 10 and 20, that's when everything really started getting real."

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The crowd scattered, fleeing for exits and taking cover. Those who couldn't escape hid in bathroom stalls, a dressing room, behind doors, under the DJ booth, and in air conditioning vents. One survivor told CNN she hid under bodies. Around 5 a.m., a SWAT team smashed down the door of the club with an armored vehicle, confronted Mateen in the doorway, and killed him.

When first responders were finally able to enter the nightclub, they had to step across the bodies strewn over the dance floor as a "cacophony of ringing unanswered cellphones" surrounded them. To locate survivors, the crew called out, "If you are alive, raise your hand."

Authorities finished removing the bodies overnight. At least 49 people were killed and 53 wounded in Sunday's attack.