• US citizen Omar Mateen opened fire inside gay nightclub
• At least 50 people killed and 53 injured in the attack
• Police used an explosive device to distract the gunman
• Mateen shot dead three hours after massacre began
• He pledged his allegiance to Isis before the massacre
• Barack Obama calls for gun control action
Orlando terrorist Omar Mateen was investigated by the FBI for terror links twice - and was linked to American suicide bomber Moner Abu Salha.
Agents did not charge Mateen on both occasions and concluded he "only had minimal contact" with Abu Salha who blew himself up in 2014.
Mateen was also investigated by the FBI for making "inflammatory remarks" to his colleagues alleging that he had terrorist ties.
He kept his job with a global security firm, G4S, and was able to legally buy guns that were used in the worst mass shooting in American history that killed 50 people inside the Orlando gay nightclub.
Mateen, from Port St Lucie in Florida, shot more than 100 people and injured 53 others with an AR-15 assault rifle and handgun on the dance floor at Pulse night club early Sunday morning.
The shooter, who had no criminal history, was licensed to work as an armed security guard in Florida, law enforcement sources said.
An FBI spokesman said at a press conference later in the day that Mateen purchased multiple guns in the past few days.
The spokesman said that the investigations by the FBI were closed and that's why the 29-year-old gunman was able to purchase the weapons.
The federal agency said Mateen was first interviewed in 2013 after he made "inflammatory remarks" to a colleague.
"The FBI first became aware of Mateen in 2013 as he made inflammatory comments to co-workers, alleging possible terrorist ties," Ron Hopper an FBI special agent said during a press conference.
"The FBI thoroughly investigated the matter including interviews of witnesses, physical surveillance and records checks.
"In the course of the investigation, Mateen was interviewed twice.
"Ultimately we were unable to verify the substance of his comments, and the investigation was closed."
In 2014, Mateen came to the FBI's attention again and agents interviewed him about a potential connection he may have had with American suicide bomber Moner Abu Salha, who lived about 30 minutes away in Vero Beach, Florida.
"We determined that contact was minimal and didn't to constitute a substantive relationship or threat at that that time," Hopper said.
In addition, the Bureau of Alcohol, Tobacco and Firearms said that Mateen purchased two weapons legally within the last week.
"He is not a prohibitive person so he can legally walk into a gun dealership,' the representative from the ATF said. 'He did so within the last week or so.'
"He's a known quantity," another source said. "He's been on the radar before."
Global security firm, G4S, confirmed that he worked for the company since 2007.
"We are shocked and saddened by the tragic event that occurred at the Orlando nightclub," a statement by the company reads.
"We can confirm that Omar Mateen had been employed with G4S since September 10, 2007. We are cooperating fully with all law enforcement authorities, including the FBI, as they conduct their investigation.
"Our thoughts and prayers are with all of the friends, families and people affected by this unspeakable tragedy."
An FBI spokesman said Sunday's mass shooting is being investigated as an act of terrorism, adding that they are looking into whether it was an act of domestic or international terror, and if the Mateen was a lone wolf.
The FBI believe Mateen, who was shot dead by officers, may have 'leanings to radical Islamic terrorism'.
But his father, Mir Seddique, told NBC News his son became angry when he saw two men kissing in Miami several months ago.
"This has nothing to do with religion," he said. "We are saying we are apologizing for the whole incident. We weren't aware of any action he is taking. We are in shock like the whole country."
Mateen, a Muslim and father to a three-year-old son, was born in 1986 in New York and married Sitora Alisherzoda Yusufiy, who was born in Uzbekistan, in 2009.
His father told NBC News: "We were in Downtown Miami, Bayside, people were playing music. And he saw two men kissing each other in front of his wife and kid and he got very angry. They were kissing each other and touching each other and he said, 'Look at that. In front of my son they are doing that'. And then we were in the men's bathroom and men were kissing each other."
Seddique said Mateen attended Indian River State College, as well as having an associates degree in criminal justice.
Manteen had a Statewide Firearms License and was trained in firearms, according to Fox News.
Hopper said Mateen made "threats in the past that he has ties to terrorist organizations".
Police were at his apartment in Port St. Lucie this morning. Ken Mascara, Sheriff of Port St. Lucie County, told DailyMail.com: "We have sealed the apartment where Mateen was living. The FBI is bringing bomb diffusing equipment."
After spending most of the day searching Mateen's Port St. Lucie home, cops and the FBI moved on late in the afternoon to the custom-built four-bedroom home a couple of blocks of north on Bayshore Boulevard where his parents live.
Neighbor, Tony Schneider, a retired deputy sheriff told Daily Mail Online that Seddique and Shahla Mateen are 'the nicest people in the world.'
"They are very Americanized, he said. "Just very good neighbors. I have known them for six years and have never had any problems with them. My heart goes out to them - of course if they were involved they can rot in hell, but I otherwise I feel so sorry for them."
Schneider said that Seddique - who he knows as Sid - can often be seen in his yard and walking up and down the street. He said he did not know his wife nearly as well.
"I believe he is in insurance, he has done well for himself and they have three daughters, all went to college, who I believe are in the medical field.
"But I didn't know the son. I saw his picture on television this morning and I recognized him, but he only came here rarely."
Mateen exchanged gunfire with an officer working around the club at 2am before going back inside, according to Orlando Police Chief John Minia.
Writing on Facebook, Pulse, which described itself as the most prominent gay club in Orlando, urged party-goers to 'get out and keep running' as bullets started to fly.
There were about 320 people inside the club at the time of the shootings and about 100 people were taken hostage.
At around 5am authorities sent in a SWAT team to rescue the hostages. Nine hero officers used a 'controlled explosion' to distract the shooter before fatally shooting him and were able to rescue about 30 hostages who were hiding in the bathroom of the club.
During the gunfire, an officer was shot, but he was saved by his helmet. It was thought that at least one hostage had been locked in a bathroom with gunshot wounds.
Mayor Buddy Dyer said in a press conference: "Many were saved by the heroic efforts of the men and women of the OPD, the Orange County Sheriffs, Seminal County Sheriff's office."
At around 6am local time police tweeted: "Pulse Shooting: The shooter inside the club is dead."
Since the news of the horrific shooting emerged, celebrities and politicians are calling for stricter gun control laws.
President Obama furthered his gun control message today when addressing the massacre in Orlando.
"Although it's still early in the investigation we know enough to say that this was an act of terror and an act of hate," Obama said, making no reference to Isis or Islamic terror in his brief remarks.