MIAMI (AFP) - The father of the gunman who killed 49 people at a gay nightclub in Orlando, Florida was spotted at a speech Democratic candidate Hillary Clinton gave to supporters in the swing state.

Seddique Mateen was caught on camera seated in the audience behind Clinton during her campaign appearance Monday night (Aug 8) in Kissimmee, a town 37km south of Orlando.

Mateen's son, Omar, proclaimed his allegiance to the Islamic State in Iraq and Syria (ISIS) group during the June 12 massacre at the Pulse nightclub, which ended after three hours when police finally stormed the venue and shot him to death.

It was the deadliest mass killing on US soil since the Sept 11, 2001 attacks on the United States.

Clinton, on a trip through the crucial swing state, opened her speech with a tribute to the 49 people who lost their lives in Orlando.

Mateen, an immigrant from Afghanistan, was approached by a reporter from the local WPTV television station and asked about his presence at the rally.

"Why should they be surprised? I love the United States, and I've been living here a long time," he said.

"Hillary Clinton is good for United States versus Donald Trump, who has no solutions," he said.

The rally was open to the public and about 3,000 people attended.

"This individual wasn't invited as a guest, and the campaign was unaware of his attendance until after the event," the Clinton campaign said in an e-mail sent to reporters.

On Tuesday, the Orange County medical examiner's office released the official autopsy reports for the victims of the Orlando shooting.

It counted 200 gunshot wounds among the dead alone. Another 53 people were wounded in the attack.