The gunman who killed 49 people after opening fire in an Orlando gay club was a homophobe and a racist who was both anti-Semitic and misogynistic according to a former co-worker.

Daniel Gilroy worked with Omar Mateen, the man responsible for Sunday's brutal massacre, at G4S Security in Port St. Lucie, Florida for several months in 2014 and 2015, during which time he observed how 'unhinged' and filled with rage the man was at all times.

Mateen would use 'horrible words' whenever he saw a 'female or black person' according to Gilroy, and on one occasion said 'I would just like to kill all those n******.'

He also frequently launched into tirades against gay people and members of the Jewish faith said Gilroy, and often talked about killing people.

Gilroy eventually complained about Mateen's 'toxic' behavior to his employer but they did nothing to remedy the situation, forcing him to request a transfer so he could get away from his co-worker.

He believes that their employer did nothing because of Mateen's religion, describing the man as a devout Muslim who brought his prayer mat to work every day.

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Not surprised: Daniel Gilroy told ABC News that his former colleague was frightening to work with

Place of employment: G4S, a British security services company (Crawley headquarters above), said in a statement that Mateen twice passed a background check when he joined the firm

'He was scary in a concerning way,' Gilroy said in an interview with NBC News.

He did not like women at all. He did like women in a sexual way, but he did not respect them.

'And it wasn't at times. It was all the time. He had anger management issues. Something would set him off, but the things that would set him off were always women, race or religion. [Those were] his button pushers.'

Gilroy added that Mateen always spoke about members of different races, religions or genders in a 'derogatory' manner.

'He did not like black people at all. That was mentioned once or twice, but more so was women,' said Gilroy.

'He did not like women at all. He did like women in a sexual way, but he did not respect them.'

Gilroy worked the community security gate at the PGA Village South in Port St. Lucie for several months in 2014 and 2015, covering the 7am to 3pm shift. Mateen would then take over for him, working from 3pm to 11pm at the resort.

'Everything he said was toxic and the company wouldn't do anything,' Gilroy told Florida Today.

'This guy was unhinged and unstable. He talked of killing people.'

Gilroy said that Mateen also began to stalk him a few months after they started working together, and that he would receive 20 to 30 text messages a day from the man and another 10 to 15 voicemails on his phone.

Mateen (above) killed 49 people in the gay club Pulse and injured 53 others before being shot dead

Those messages, coupled with Mateen's behavior and beliefs, ultimately led to Gilroy requesting a transfer he revealed as he was unable to continue working with the man.

Gilroy claims that prior to that he voiced his issues with Mateen to their employer on several occasions, but that they refused to do anything about the situation.

He believes that G4S never took action because Mateen was Muslim.

G4S issued a statement on Sunday admitting that Mateen twice passed a background check when he joined the firm in September of 2007 and that he was not fired because the FBI interviewed him about links to Islamic extremism.

They also said that they are now 'cooperating fully with all law enforcement authorities, including the FBI, as they conduct their investigation.'

These damaging revelations come as shares in G4S, which is listed on the London Stock Exchange, lost up to eight percent of their value during trading on Monday.

The world's biggest security company said he worked as a staff member at a gated retirement complex called PGA Village, in Port St Lucie, South Florida.

It is not the first time the FBI is looking into Mateen, who was a 'known quantity' to federal law enforcement officials.

I'm not surprised at what he did. I always knew he had something in him like that. And if I had to pick one person that would be capable of that, it would be him.

In 2013, Mateen was interviewed twice by the FBI after making comments to co-workers that seemed to indicate sympathy for militants according to agent Ronald Hopper.

Cooper called those interviews inconclusive.

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

Once again however the FBI closed its inquiry after finding ‘minimal’ contact between the pair.

Those two interviews by the FBI and their inquiry into his relationship with Abusalha all occurred in the span of just two years, yet Mateen still managed to keep his security job with G4S.

He was also able to legally purchase the two guns he used in his attack, including an AR-15 semi-automatic rifle which he bought just last week.

Gilroy said he lost contact with Mateen in March of last year when he left PGA South and is not shocked by the news that he killed 49 people and injured 53 at the gay club Pulse in Orlando on Sunday morning before being shot dead by police.

'When I say that he was unstable or unhinged, I mean this is a man who would lose his temper for no reason,' Gilroy said to ABC News.

'He would kick walls, slap desks. I've seen him throw the chair across the room one time.'

Gilroy also admitted he was glad to never see Mateen again.

'I didn't want to be around him anymore,' said Gilroy.

'I'm not surprised at what he did. I always knew he had something in him like that. And if I had to pick one person that would be capable of that, it would be him.'

Gilroy revealed he is now wondering though if he could have done more to have somehow stopped this violent attack.

'I kind of feel a little guilty that I didn’t fight harder,' said Gilroy.

'If I didn’t walk away and I fought, then maybe 50 people would still be alive today.'

Former love: Sitora Yusufiy (above on Sunday), Mateen's ex-wife, spoke about his violent temper in an interview shortly after the shooting

Rage: Mateen's father, Seddique Mir Mateen (above), said on Sunday that his son was homophobic, recalling an incident that happened years ago in Miami.

Other individuals who were close to Mateen seem to confirm many of the claims being made by Gilroy about the man's behavior.

Mateen's father, Seddique Mir Mateen, said on Sunday that his son was homophobic, recalling an incident that happened years ago in Miami.

'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,' said the elder Mateen.

'They were kissing each other and touching each other, and he said: "Look at that. In front of my son, they are doing that."'

He also stated that the brutal massacre committed by hos son had 'nothing to do with religion.'

Sitora Yusufiy, Mateen's ex-wife, spoke about his violent temper in an interview shortly after the shooting.

'He would be perfectly normal and happy, joking, laughing one minute. The next minute his temper ... his body would just [go] totally the opposite,' said Yusufiy, who married Mateen in 2009 and filed for divorce just two years later.

'Anger, emotionally violent - and that later evolved into abuse, to beating.'

Yusuify, who claims she was abused throughout her relationship with Mateen, also said that he was not a stable person.

There are some however who are painting a far different picture of Mateen, with one resident at the PGA Village South saying Mateen was 'always smiling' and 'a very nice, positive person.'

Eleanora Dorsi, who has lived at the resort since 2011, said that Mateen was a gentleman and 'not creepy,' mentioning that he once helped her fix her car when she needed a hand.

'You would never ever think that he would have done anything like this,' said Dorsi.

'Scary, scary. Very scary. I think everyone feels like that right now.

'It hits too close to home.'

A former classmate of Mateen's who is openly gay and knew him before 2009 also said that he regularly interacted with him and other homosexuals and did not seem to have any issue with their sexuality.

He 'treated us all like the individuals we were' said Samuel King.

'He clearly was not anti-(gay) at least not back then. He did not show any hatred to any of us.'