Adam Shell

USA TODAY

A night of partying and dancing at Pulse, a gay nightclub in Orlando, turned into a killing ground and three hours of terror for early-morning revelers when a gunman opened fire around 2 a.m. ET inside the club, killing 50 people and wounding at least 53 in what is being called the worst mass shooting in U.S. history.

Here’a a rough timeline of the three hours from the time the gunman gained entrance to the club, Pulse Orlando, around 2 a.m. ET to the time authorities killed the assailant around 5 a.m. ET. (All times are Eastern standard time).

SIEGE BEGINS:

2:02 a.m. Shots ring out as a police officer working as a security guard engages in gun battle with assailant, Omar Mateen, 29, of Port St. Lucie, Fla., outside the Pulse Orlando nightclub around closing time, according to Orlando police chief John Mina. Mateen was armed with an AR-15-style assault rifle, a handgun and an unidentified device.

The gunman makes way into club, where more than 300 people were enjoying Latino night, and opens fire, causing chaos as patrons realize what is unfolding. Mateen fired an "unknown number of rounds," the police chief said. "There were numerous shots." Eyewitnesses said people on the dance floor and around the bar area dived to the floor, with some lucky enough to flee.

Jackie Smith, who was inside the club and says two friends were shot, told the Associated Press after release from Orlando Regional Medical Center: "Some guy walked in and started shooting everybody. He had an automatic rifle, so nobody stood a chance. I just tried to get out of there."

2:06 a.m. Mina Justice was sound asleep when, she told the Associated Press, she got a text from her son, Eddie, from inside the club.

“Mommy I love you,” the message said. “In club they shooting.”

2:09 a.m. About seven minutes after the first shot, the Pulse Orlando nightclub's Facebook page zapped out a chilling message: "Everyone get out of pulse and keep running."

2:17 a.m. @bjoewolf (Brandon Wolf), tweets: “Omg. Shooting at pulse. We hid in the bathroom. And we can’t find our friends."

Club goer Christopher Hansen who escaped the club, later told CNN, “It’s just shocking, I just saw bodies going down.” Hansen estimates that 20 or 30 or even more shots were fired.

Speaking to reporters after the assault, Florida Rep. Alan Grayson, a Democrat, said: "All the killing that was done was with an assault rifle, a single weapon...it was done very quickly also."

MULTI-HOUR HOSTAGE SITUATION ENSUES:

2 a.m. ET to 5 a.m. ET. Law enforcement and the shooter engage in three-hour standoff, with the assailant holding hostages.

Updates and pleas for help from frightened patrons, many hiding in a restroom, trapped in the club were zapped out to police, friends and the world via text messages and social media, such as Twitter.

The gunman, according to the FBI, made a call to 911 during the attack and pledged his support to the Islamic State, or ISIS.

Multiple emergency vehicles and law enforcement teams respond to the scene of the shooting. Bomb units were sent to the scene, as were hazardous material teams.

Around 3 a.m. Orlando Fire Department calls in the bomb squad. SWAT teams were also called in.

3:58 a.m. Orlando police, via Twitter, warn public to stay away from the nightclub area.

Law enforcement officials say there was "some communication" with assailant, but would not provide details. "Once the initial shots stopped we were dealing with a hostage situation," Mina said in a press conference.

Orlando Mayor Buddy Dyer says authorities learned through text messages from hostages in the club that between five and eight hostages were with the shooter in a room, while 15 to 25 others were in another room elsewhere in the nightclub.

SIEGE ENDS AS POLICE STORM CLUB, KILL ASSAILANT

5 a.m. About three hours after the standoff began, Orlando police chief John Mina says the decision was made to storm the club and "rescue patrons (hiding) in a bathroom" and others still trapped inside the club in an effort to minimize loss of life. The police, Mina said, used a BearCat armored rescue vehicle to "get through a club wall."

Cops also set off a "controlled detonation" around this time to distract the assailant.

After the BearCat rammed the club, the shooter opened the club's door, according to Mayor Dyer, and that's when cops took aim at the shooter.

Gunfire was "exchanged between the SWAT team and the suspect at 0500 hours," or 5 a.m., officials said later during a press conference.

Police said the gunman was killed in the firefight.

“We exchanged gunfire with the suspect, and he was dead at the scene," Mina said, adding that the SWAT team's decision to go in saved the lives of "at least 30 possible victims."

5:53 a.m. About an hour later the Orlando police department confirm via Twitter that the assailant, Mateen, had been killed: “Pulse Shooting: The shooter inside the club is dead.”