Omar Mateen, the man who killed more than 50 people in an Orlando, Florida, nightclub on Sunday, was apparently an ISIS supporter.
According to law enforcement sources that spoke with NBC News, Mateen called 911 moments before he shot more than 100 people at Pulse Nightclub and pledged allegiance to ISIS.
Mateen, 29, reportedly mentioned the Tsarnaev brothers -- who carried out the 2013 Boston Marathon bombings -- at some point during the call and referenced ISIS propaganda.
Earlier in the day, Mateen’s father claimed that his son’s rampage was the result of him seeing two men kissing and his homophobic feelings taking over. However, police are now looking closely at the theory that radicalization was behind the attack.
Mateen was born in New York and lived in Port St. Lucie on Florida's eastern shores. The FBI had previously investigated him, but found no cause for arrest.
On Sunday afternoon, President Obama referred to what transpired at Pulse nightclub as an "act of terror and an act of hate."