Mateen’s father reportedly said the attack Sunday morning at a popular gay nightclub in Orlando, Florida, “has nothing to do with religion.”
He instead said his son had recently become very upset by the sight of two men kissing in downtown Miami, and pointed to his anger as a possible cause.
“We weren’t aware of any action he is taking,” Mateen’s father said. “We are in shock like the whole country.”
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Witnesses have described the shooting scene to PEOPLE, from both inside and around the nightclub, as “crazy” and “awful.”
Authorities, who have not named the suspect, have said they do not yet know a motive.
An FBI spokesperson said Sunday morning that this is an ongoing investigation and wouldn’t classify it as a terror or hate crime at this time.
However, an FBI source with knowledge of the investigation tells PEOPLE, “Agents in Afghanistan are looking at the shooter’s connections there.”
The source adds, “He has family there. This is not to say his family is connected to the shootings. This is the normal course of an investigation like this. We ask questions.”
The source adds, “We are examining the shooter’s social media activity. Did he have connections to radical factions? Was he inspired?”
“This is a priority,” the source adds. “The Islamic State called for attacks on the West during Ramadan. In the wake of San Bernardino, we are very much mindful of how this can play out.”
• Reporting by SUSAN KEATING