Other patrol officers rush into the club during the shootout to pull survivors out.
The shooter retreats and after exchanging gunfire with a SWAT lieutenant in a hallway he barricades himself and clubgoers in a bathroom.
Some 100 officers from the Orange County Sheriff's Office and the Orlando Police Department respond to the chaotic scene over the next 45 minutes.
2:05 a.m. ET: Patience Carter says the shooter enters the bathroom and shoots several people hiding there. People scream and scramble as blood flies. Then the gunman stops shooting.
2:06-2:07 a.m. ET:
Shooting victim Eddie Jamoldroy Justice texts his mother
from a bathroom stall:
--Mommy I love you
--In club they shooting
--Trapp in bathroom
Police say they receive communications from from clubgoers who say they are trapped inside the bathroom. This is when they say they knew there were hostages.
2:09 a.m. ET: Pulse posts an urgent message on Facebook: "Everyone get out of Pulse and keep running."
A tense standoff follows. Police later say they had to wait three hours to assess the situation, get armored vehicles on the scene and make sure they had enough personnel.
Approximately 2:22 a.m. ET: Shooter talks to 911 dispatch three times. He calls and hangs up. He calls and talks briefly. The dispatcher calls back. During the calls, the shooter pledges allegiance to ISIS. He also mentions the Boston Marathon bombers and a Florida man who died as a suicide bomber for al Nusra Front in Syria. Patience Carter says he also told a 911 dispatcher he wanted America to stop bombing his country. She said he started speaking a foreign language, probably Arabic, when he ended the call.
Approximately 2:45 a.m. ET: Killer calls News 13 Orlando and speaks to producer Matt Gentili. Shooter takes responsibility for the attack and claims he did it for ISIS. At some point, he also posts on Facebook.
2:49-2:51 a.m. ET: Justice texts his mother again:
--He;s in the bathroom with us
--women's bathroom is
--(mom) is the man in the bathroom wit u
--hes a terror
Justice was killed.
Approximately 2:30-5 a.m. ET: Hostage negotiators communicate with the shooter while he's barricaded in the bathroom. He threatens to put explosive vests on four hostages and to detonate the vests in 15 minutes. SWAT officers prepare for an explosive breach. During this time police do not exchange fire with the killer.
Officers continue to rescue people from inside the club while the gunman is in the bathroom.
While officers are negotiating with the suspect, eight people were able to escape with the help of law enforcement, by removing an air conditioning unit and crawling through the hole created.
3:58 a.m. ET: A post on Orlando Police Department's official Twitter account says: "Shooting at Pulse Nightclub on S Orange. Multiple injuries. Stay away from area."
Around 4 a.m. ET: The killer texts his wife and asks if she has seen the news, a source tells CNN.
Approximately 4 a.m. ET: Orlando's chief medical examiner, Dr. Joshua Stephany, receives a call about 11 or 12 casualties in the club. Soon after, he receives notification that the body count had reached 20, then 30. Stephany and his staff head to the nightclub.
4:57 a.m. ET: About 40 patients have already been transported to Orlando Regional Medical Center, according to Orlando fire dispatch audio.
Approximately 5 a.m. ET: SWAT members detonate an explosive to blow a hole in the wall but its only partially effective. They then use an armored vehicle to breach the final hole in the wall. But it leads them only to the hallway, not to the bathroom. They continue trying to breach using the vehicle.
They also smash down a door at the club, clearing the way for some 30 people inside to flee to safety.
5:05 a.m. ET: Post on Orlando police Twitter account says loud noise near scene was a "controlled explosion."
Approximately 5:15 a.m. ET: CNN field team and affiliates report hearing multiple gunshots.
As SWAT officers attempt to breach another wall, the suspect leaves the bathroom and he begins shooting at them. Police return fire and kill him.
5:53 a.m. ET: Twitter account of police posts that shooter is dead.
7:15 a.m. ET: Police Chief John Mina announces at a news conference that multiple people have died inside the club. He says the shooter was armed with an assault-type rifle and a handgun.
10:15 a.m. ET: Authorities identify the shooter as Omar Mateen, 29, of Fort Pierce, Florida.
12:03 p.m. ET: Gov. Rick Scott posts on Twitter: "This is an attack on our people. An attack on Orlando. An attack on Florida. An attack on America. An attack on all of us." He declares a state of emergency in Orange County.
1 p.m. ET: President Barack Obama addresses the attack from the White House, calling it an act of terror and an act of hate.
The shooter was a "person filled with hatred," Obama says. He also orders the American flag be lowered to half-staff at the White House and all other government buildings until sunset on June 16.
"In the face of hate and violence, we will love one another," Obama says. "We will not give in to fear or turn against each other, instead we will stand united as Americans to protect our people, to defend our nation, and take action against those who threaten us."