The presence of Seddique Mateen -- whose controversial views on Afghan politics and homosexuality were made public in the aftermath of the June massacre at Pulse nightclub -- and his proximity to a presidential candidate, raised eyebrows.
Seddique Mateen was seated in an area directly behind Clinton, within view of cameras broadcasting the event.
A U.S. Secret Service official declined to discuss Mateen specifically, but said that "the roughly 3,000 participants at the Clinton rally in Florida all received the exact same screening."
Security at public campaign rallies includes metal detectors, but attendees are typically not required to undergo background checks or receive additional scrutiny unless they will be sitting within feet of the candidate. These seats are usually reserved for invited guests.
On Clinton's campaign website, the only information requested in the RSVP for the Kissimmee rally was a first and last name, email address, phone number and ZIP code. An RSVP was not required for the event and it was unclear if Mateen RSVP'd.
Though on television it appeared that Mateen was close to Clinton, he was well outside of the security buffer zone in a location that a longtime Republican presidential campaign advance staffer described as a "tapestry" -- an area where a diverse group would be seated to reflect wide-ranging support for the candidate.
"A presidential campaign should take a more cautious approach to selecting who's in the shot, but accidents happen," the Republican staffer told ABC News.
Seddique Mateen was not investigated for any wrongdoing in connection to June's shooting. As for his political beliefs, he told NBC West Palm Beach affiliate WPTV that he supports Clinton.
"Hillary Clinton is good for United States versus Donald Trump, who has no solutions," he said.
Following the event, the Clinton campaign released a statement regarding Mateen's presence.
"The rally was a 3,000-person, open-door event for the public. This individual wasn't invited as a guest, and the campaign was unaware of his attendance until after the event."
ABC's Shushannah Walshe and Devin Dwyer contributed to this report.