Here are the pro-ISIS messages the Orlando shooter posted on Facebook during the attack
He killed 49 people in the attack and said he did it in the name of ISIS, the group also known as the Islamic State, ISIL, or Daesh.
Omar Mateen, 29, was on a terror watch list in the US after authorities questioned him in separate 2013 and 2014 investigations. He was known to authorities because he had expressed sympathies for other terror groups, including Al Qaeda affiliate Jabhat al-Nusra and Shia militant group Hezbollah, in the past.
CBS News obtained one of Mateen's Facebook messages, which reportedly read:
You kill innocent women and children by doing us airstrikes..now taste the Islamic state vengeance [sic]
In the next few days you will see attacks from the Islamic State in the usa.
Fox News reports that he posted other messages after the attack started. Senate Homeland Security Chairman Ron Johnson shared the messages with Fox. They reportedly read:
I pledge my alliance to [ISIS leader] abu bakr al Baghdadi..may Allah accept me
The real muslims will never accept the filthy ways of the west
Mateen also searched for the terms "Pulse Orlando" and "shooting" on Facebook during the attack, according to Fox.
Johnson, a Republican from Wisconsin, sent a letter to Facebook on Wednesday asking the company to assist in the investigation of the shooting.
Mateen died in a shootout with police hours after the attack began. He also pledged allegiance to ISIS in a 911 call during the attack. An FBI source told Fox that Mateen made a total of 16 phone calls from inside the club.
And a local TV station in Orlando reported taking a call from someone who claimed to be the nightclub shooter while the attack was still ongoing. The caller said he was the shooter and then added: "I did it for ISIS. I did it for the Islamic State."
Despite Mateen's pro-ISIS statements, his motives likely go beyond just terrorist ideology. Some of the people closest to Mateen have hinted at psychological issues, and witnesses have come forward saying Mateen frequented the nightclub he attacked.