The FBI states that it found no evidence of him being gay or any of the purported apps in his digital records, and considered the accounts of his purported gay lovers to be non-credible. For example, Mateen may have used the app to look for victims and visited the club in order to best plan out his course of attack. Some have claimed that Mateen was a self-hating gay man, but there may be other reasons for these reports. It is also claimed he used and messaged other gay men via a gay dating app. Later reports state Mateen had frequented the Pulse nightclub in the past with him being recognized by several regulars. After the shooting, it was revealed that he had been on the FBI's radar since 2013. He was killed in a shootout with SWAT officers at around 5:15 AM. Mateen called law officials, pledged allegiance to ISIS and Hezbollah (despite the two groups fighting against each other in Syria), praised the Boston bombing, and was otherwise remorseless about the slaughter. Months later, he traveled two hours to get to the club, before committing the shooting with an MCX semi-automatic gun at around 2:00 AM.

According to his father, he became very angry when he witnessed two men kissing.