Seddique Mateen attended a Clinton campaign rally in Kissimmee, Fla., on Monday, WPTV reported.
He spoke about his support for Clinton at a rest stop in West Palm Beach, Fla.
“It’s a Democratic Party, so everyone can join,” Mateen told the network.
He defended his attendance at the Clinton rally, saying, “I love the United States, and I’ve been living here a long time.”
“I spoke a lot about that and wish that my son joined the Army and fought [the Islamic State in Iraq and Syria],” Mateen added. "That would be much better.”
The Clinton campaign told ABC News that Mateen was not invited to the Kissimmee event.
“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.”
Clinton began her rally in Kissimmee by remembering the 49 people killed and 53 others wounded at Pulse nightclub in June.
“I know how many people, families, loved ones and friends are still grieving, and we will be with you as you rebuild your lives,” she said.
Omar Mateen, 29, opened fire on clubgoers on June 12 in what would become the deadliest mass shooting in U.S. history. He was shot and killed by responding police officers.

 Updated at 10:33 a.m.