The shooter who massacred at least 49 people inside a Florida gay nightclub is believed to be connected to a former US Marine who turned into a radical Muslim cleric that was released from prison last year.

Omar Mateen, who was shot dead by a SWAT team who stormed inside the Pulse nightclub early Sunday morning, is believed to be a radical Muslim who followed Marcus Dwayne Robertson.

Robertson, who is also known as Abu Taubah to his thousands of followers, converted at least 36 people to his version of Islam while he was imprisoned for four years in jail.

Early Sunday authorities brought Robertson, who was living in Florida, along with several other associates in for questioning.

According to Fox News, a law enforcement source said: 'It is no coincidence that this happened in Orlando. Mateen was enrolled in (Robertson's online) Fundamental Islamic Knowledge Seminary.'

Police believe that Robertson's Timbuktu Seminary is used to dispense his radical teachings, sources told Fox News.

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Connected?: Omar Mateen (left) is believed to be connected to Marcus Dwayne Robertson (right), a former US Marine who turned into a radical Muslim cleric that was released from prison last year

Mateen, 29, the Islamic extremist who slaughtered at least 49 people and injured dozens more inside Pulse nightclub in Orlando early Sunday morning. The FBI interviewed Mateen twice in 2013 and a third time in 2014

Mateen was a member of the Timbuktu Seminary, an educational website run by Robertson (above) that police believe is used to dispense his radical teachings

Robertson, who was a former US Marine and undercover FBI agent before turning into a radical Imam, was released from prison last year despite warning from officials and prosecutors that he would recruit people to carry out horrendous acts of violence.

He was considered so dangerous that he was kept shackled with his own security detail away from other inmates.

Whenever he was transported to court, authorities had a seven-car caravan of armed federal marshals escort him.

He was eventually moved to solitary confinement after prison officials discovered that he was trying to radicalize his fellow inmates.

After serving as a Marine for six years, Robertson, 47, went on to become a bank robber, before turning FBI informant after his arrest in exchange for a short prison sentence.

He was dismissed by the FBI in 2007 after allegedly attacking his CIA handler, and then began preaching Islamic extremism.

Robertson (pictured), who is also known as Abu Taubah, converted 36 people to his version of Islam while he was imprisoned for four years in jail. He was brought in for questioning early Sunday

The FBI interviewed Mateen in 2013 and closed the investigation and then opened another in 2014 when he came to the FBI's attention again. Agents interviewed him about a potential connection he may have had with American suicide bomber Moner Abu Salha (pictured above)

Authorities say both Mateen and Abu Salha attended the same mosque, Fort Pierce Islamic Center (pictured), in Orlando, Florida

He was thrown in jail for tax fraud back in 2011, prosecutors attempted to have ten years added to his sentence last year after discovering documents preaching terror among his possessions. However, a judge released him.

Robertson has openly preached against homosexuality, as wiretaps from 2011 proved that he instructed one of his students to file fake tax returns to get a refund to pay for travel to Mauritania for terror training.

According to Fox News, federal law enforcement officials have been familiar with Robertson since as early as 1991.

Robertson became the leader of a New York gang known as 'Ali Baba and the 40 Thieves' and robbed more than 10 banks, private homes and post offices at gunpoint.

They shot three police officers and also attacked an officer after he suffered injuries from a homemade pipe bomb.

In that same period, authorities claimed that Robertson, who was known as 'Ali Baba,' also 'served as a bodyguard to Omar Abdel Rahman, who led the terrorist group that carried out the 1993 bombing of the World Trade Center,' Fox News reported.

When he was arrested in 1991 he cut a deal with prosecutors who let him serve four years in prison before he went to work undercover for the FBI in 2004 to document terrorists' plans in Africa and the United States.

Attack: A woman sits on the ground outside the club while another party-goer, whose legs are covered in blood, stands beside her

Mateen, from Port St Lucie in Florida, shot more than 100 people, killing 49 and injuring 53 others with an AR-15 assault rifle and handgun on the dance floor at Pulse nightclub early Sunday morning.

The shooter, who had no criminal history, was licensed to work as an armed security guard in Florida for global security firm G4S.

In addition, Mateen was also at the Islamic Center of Fort Pierce with Imam Shafiq Rahman two days prior to the horrific nightclub shooting, The Washington Post reported.

Rahman told the Post he didn't know Mateen well, but he brought his son with him two days before the shooting to the mosque.

'He was the most quiet guy, he never talked to anyone,' Rahman said. 'He would come and pray and leave. There was no indication at all he would do something violent.'

In 2014, the FBI discovered a possible tie between Mateen and Abu Salha, who had grown up in nearby Vero Beach and became the first American suicide bomber in Syria, where he fought with the Nusra Front.

The FBI closed its inquiry after finding ‘minimal’ contact between the pair, though they frequented the same mosque.

In 2013, Mateen made inflammatory comments to co-workers, and was was interviewed twice, FBI agent Ronald Hopper said. He called those interviews inconclusive.

Response: Emergency services are pictured at the scene outside the Pulse nightclub in Orlando, Florida following Sunday morning's shooting

Mateen 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.

Once inside Pulse nightclub, Mateen was involved in three calls with a 911 dispatcher at approximately 2:30am.

Director of the FBI James Comey said in a statement Monday: 'During the calls he said he was doing this for the leader of ISIS, who he named, and pledged loyalty to. But he also appeared to claim solidarity with the perpetrators of the Boston Marathon bombings and solidarity with the Florida man who died as a suicideb bomber in Syria for Al-Nusra Front - a group in conflict with the so-called Islamic State.

'The Boston bombers and the suicide bomber from Florida were not inspired by ISIS, which adds a little bit to the confusion about his motives', Comey concluded.

The shooter, who is 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.

Mateen's father, Seddique Mateen, 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 his son 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 when he legally purchased the guns used in the mass shooting.

An FBI spokesman said that Mateen made 'threats in the past that he has ties to terrorist organizations'.