Inside the Orlando Nightclub Massacre, Breaking Down the Three Horrific Hours Inside

— -- Eyewitnesses described a scene of complete chaos, with blood everywhere, in the three horrific hours from the time an armed gunman opened fire inside an Orlando, Florida, gay nightclub to the time a SWAT team entered the building, rescued the hostages the gunman had taken and then killed him.

One eyewitness who was released from the hospital tonight told ABC News he played dead for three hours as the gunman, whom authorities have identified as Omar Mateen, 29, walked around, talking to club patrons still inside. Then, the witness said, he heard the shooter on the phone saying, "Stop killing ISIS" and "I've got plenty of bullets."

By the time the attack was over, at least 50 people had been killed and at least 53 others were injured.

At 2 a.m. Sunday, there were about 320 people at the Pulse nightclub, officials said. Pulse bills itself on its website as "Orlando's premier gay night club."

At 2:02 a.m., bartenders announced last call, then, law enforcement officials said, Mateen, armed with an assault rifle, a handgun and multiple rounds of ammunition, walked in.

"The music was playing, everyone was dancing ... and that's where you heard the 'bang, bang,'" said Christopher Hansen, who was inside the club at the time. "The one guy next to me fell, and there was blood everywhere."

A uniformed off-duty police officer working security at the club responded. He and two other officers exchanged gunfire with the suspect, according to officials.

"Our officer engaged in a gunbattle with that suspect," Orlando Police Chief John Mina said. "The suspect at some point went back inside the club. More shots were fired."

The club has three main areas: a main bar and dance floor, a hip-hop room and a patio area. Some club patrons tried to take cover, and others escaped out a side door and into an alley.

"We are literally trying to hide underneath the bar," said Janiel Gonzalez, another eyewitness. "People are screaming. They are crawling on the floor ... I just remember telling myself, 'This is not how I die.' Like, 'Not today, it's not today' and 'not like this, at least.'"

Both Hansen and Gonzalez said they dropped to the floor and tried to crawl to the nearest exit amid the chaos.

"All I know is to just get out, so when I dropped, I crawled out," Hansen said. "And I was crawling. I just kept crawling."

"It was complete mayhem," Gonzalez said. "Everybody was just so, so nervous, and we wanted to get out. People were getting trampled. We were just trying to push everybody out the door."

By 2:09 a.m., the nightclub posted a chilling warning on its Facebook page, saying, "Everyone get out of pulse and keep running."

When he got outside, Gonzalez said, he could see people covered in blood. Hansen said there were "bodies everywhere."

"In the parking lot they were tagging them red, yellow, so that they know who to help first and who not to help first," he continued. "There was pants down, shirts cut off. They had to find bullets. There was just blood everywhere."

People continued to try to escape, but dozens were stuck and became hostages, law enforcement officials said at a news conference earlier today. After the shooting began, Mateen, a U.S.-born citizen with Afghan parents who lived in St. Lucie County, Florida, called 911 to pledge his allegiance to the terrorist group ISIS, officials said.

Armored vehicles and a SWAT team rushed to the scene but then waited outside the club for nearly three hours.

Danny Banks, the Florida Department of Law Enforcement's special agent in charge, said at a news conference, "In that time, we need to set up, re-evaluate, reassess what's happening and make sure that all the pieces were in place."

At approximately 5 a.m., the SWAT team made the decision to rescue the hostages, officials said. Police set off controlled explosions inside the club to distract the gunman. An armored vehicle smashed through a wall of the club, and officers swarmed in behind. Eleven Orlando Police officers exchanged gunfire with the shooter, according to Mina.

"We made a decision to do an explosive entry on the outside of the club to get to those people, and our SWAT team at that time encountered the suspect somewhere in that area, near one of the doorway entrances, and shot and killed him," Mina said.

At 5:53 a.m. Orlando police tweeted, "The shooter inside the nightclub is dead."

The massacre at Pulse has been described as a domestic terrorism incident. It is the worst mass shooting in U.S. history and the deadliest terrorist attack on U.S. soil since Sept. 11, 2001.