Chris Callen, a performer at Pulse in Orlando, Florida, told the New York Daily News that Omar Mateen had visited the venue over the past three years.
Investigators are pursuing the reports, according to US media.
Mateen opened fire in the packed club early on Sunday, leaving 49 people dead and dozens wounded.
Police shot the 29-year-old dead after he barricaded himself in a bathroom, taking several people hostage.
"I`ve seen him a couple of times at Pulse, a couple of other people that I`ve spoken with, including an-ex security guard, have actually witnessed this guy at Pulse many times before," said Chris Callen.
In one incident, Mr Callen said Mateen pulled a knife on a friend after being angered by a religious joke.
Despite this, Mr Callen said Mateen was a "nice guy... Maybe he got radicalised and hated who he was."
"Sometimes he would go over in the corner and sit and drink by himself, and other times he would get so drunk he was loud and belligerent," Ty Smith told the Orlando Sentinel.
And another man, Kevin West, told the Washington Post he saw Mateen walking into Pulse in the early hours of Sunday, having already met him through dating app Jack`d.
"I remember details," said Mr West. "I never forget a face."
While there has been no official comment from the authorities on the claims, CNN and NBC said investigators were examining them.
Orlando killer Omar Mateen `visited Pulse gay club`
14 June 2016
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The man behind the worst mass shooting in recent US history visited the gay nightclub where he carried out the massacre several times, witnesses say.
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