The images show forensic officers examing a huge holes punched into the building by bullets and explosives - possibly used as a diversion by a SWAT team who stormed the club and killed Mateen after he took some clubbers hostage
Witnesses described how lone-wolf gunman Mateen, 29, indiscriminately massacred partygoers at the gay nightclub.
One reveller texted his mother: “I’m going to die”, as the maniac–armed with an assault rifle and pistol – made his way into the venue, and claimed at least 50 lives.
Orlando shooting investigators had to block out the sound of mobiles constantly ringing as the bodies lay where they fell.
According to CNN reports, "Investigators were forced to 'tune out' sound of ringing phones coming from bodies strewn about the scene of the massacre.
Islamic State claimed responsibility for the atrocity, which officials said was the “worst terrorist attack on American soil since 9/11”.
Mateen, 29, is said to have been involved in a gunfight with a police officer outside the club at about 2am, before entering the building, killing many before taking hostages.
The club’s owner quickly took to Facebook pleading with everyone to escape, warning: “Everyone get out of Pulse and keep running.”
Partygoer Eddie Justice texted his mother Mina from the toilets inside the club as she waited outside.
He wrote: “He’s coming. I’m gonna die,” before then saying: “He has us, and he’s in here with us.”
It was the last she heard from him and his condition is not yet known.
Another reveller, Kenneth Melendez, said four of his friends were shot and taken to hospital.
He said: “At first, when I heard the shots I thought it was part of the music, but then we realised it was really happening. I started running and saw someone bleeding from the arm and I was like, wow, this is really happening and I kept running.”
Witness Juan Rivera added: “Around 2am when the club was about to close we heard shooting outside – once they finished the shooter came inside and he had a rifle and he started shooting everywhere. We heard about 30 shots and all of a sudden the lights went off and we had blood all over our shirts.” Mr Rivera said the gunman, who was thought to be wearing a suicide vest, said nothing and was firing as he blocked one of the two exits from the club.
He continued: “People started jumping on top of him and passing him or into the back and jumping the fence like I did and my friends did. After we escaped that is when we heard he held everyone hostage inside.” Christopher Hansen added: “It was tragic. I wasn’t sure if it was the music, because ... it went with the beat almost until you heard just too many shots.”
About 320 partygoers were inside the club. Mateen held dozens hostage between 2am and 5am before police chiefs gave the order for a heavily armed SWAT team to carry out a rescue mission.
Nine officers used an armoured vehicle to storm the club before using an explosive device to distract the killer. Mateen was shot dead. He is said to have “come on to the radar” of police but was not the target of an active investigation.
Orlando Police Chief John Mina said around 30 people were “saved during the rescue”.
Mina said an officer was injured in the eye after being shot in the head. He added: “It appears his helmet saved his life.”
A hero doorman saved dozens of lives by knocking down a partition between the club and an area in the back where only workers are allowed. Fifty-three people were taken to hospital where doctors described some of the injured as being in a critical condition.
Authorities are now investigating Mateen’s motives.
Islamic State’s Amaq news agency claimed the terror group was behind the shooting. It said: “The armed attack that targeted a gay night club in the city of Orlando in the American state of Florida which left over 100 people dead or injured was carried out by an Islamic State fighter.” Mateen, of Port Lucie, Florida, also reportedly called 911 before the attack to claim allegiance to IS. However, his father insisted his son had been motivated only by extreme homophobia.
US Senator for Florida, Bill Nelson said: “Intelligence sources believe there is a connection to ISIS, but this is not official.
“This is obviously a mass shooting incident and an act of terrorism.”
Both presidential candidates posted tweets following the massacre. Donald Trump, who has called for all Muslims to be banned from entering the US, said: “Really bad shooting in Orlando.
“Police investigating possible terrorism.” Hillary Clinton wrote: “Woke up to hear the devastating news from FL. As we wait for more information, my thoughts are with those affected by this horrific act.”
Previously the worst mass killing was the Virginia Tech massacre which left 32 people dead.
Student Seung-hui Cho opened fire on university campus before committing suicide.
Orlando is a popular holiday destination for Britons and the Foreign Office is working with US authorities to establish if any Britons were involved.
Foreign Secretary Philip Hammond tweeted: “Shocked by shooting in #Orlando. More than 50 killed or wounded. FCO in contact w/local authorities. Thoughts are with victims & families.”