A gunman who opened fire in an Orlando gay club and killed 49 people last month had complained of anti-Muslim taunts in his job as a security guard, new records reveal.

When Omar Mateen first started working at the St Lucie Court, he wrote in a statement to his bosses at G4S Secure Solutions, that one guard told deputies that "I'm a Muslim extremist and potential terrorist", according to the records released by the St Lucie County Sheriff's Office on Monday.

In one incident, a deputy teased Mateen that his fingers had pork oil on them and he was going to rub them on Mateen's shirt, according to Mateen. Observant Muslims don't eat pork because it is considered unclean.

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Another time, when Mateen was delivering boxes to the court, another guard often said: "We have to be careful, Omar may send us a bomb and he will get his 72 virgins", he wrote.

In June, Mateen opened fire at the Pulse nightclub in Orlando, killing 49 people and wounding 53 others, before being shot dead by police. He had called 911 during the attack to express his allegiance to the Islamic State of Iraq and the Levant group (ISIL, also known as ISIS).

Mateen told his bosses he responded to the taunts by claiming he had ties with the Boston Marathon bombing suspects and Nidal Hasan, a former US Army major who was convicted and sentenced to death in a 2009 shooting at Fort Hood in Texas that left 13 people dead and 31 wounded.

That prompted an FBI investigation in 2013, but it was later determined that he did not pose any threat.

The FBI investigated Mateen again in 2014 because of his ties to a Syrian suicide bomber who went to the same mosque, but that case also was closed without the agency taking action.

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Mateen was born in New York and his family came from Afghanistan. In the letter to his employer, he expressed his patriotism to the US and said he loved his job.

"I love the United States. The boasting I did it just to satisfy the gang of co-workers who ganged up against me," Matten wrote.

"I'm 1,000% pure American. I'm against these terrorists anyone of them."

The attack on the Pulse nightclub was the deadliest mass shooting in modern US history.

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Source: Agencies