CBS News obtained the alleged message Mateen posted to social media before his attack.
It was reported earlier that Mateen declared allegiance to ISIS and leader Abu Bakr al-Baghdadi in 911 calls made during the attack early Sunday morning. Mateen also made calls to local Orlando news stations saying that he committed the shooting for the Islamic State and then began speaking Arabic.
"At the time, I didn't know what he was saying," Matthew Gentili, the producer at News 13 who received Mateen's call, said. "He was speaking so fast. But it was ... he was speaking fluently. Whatever language he was speaking, he knew it. And he was speaking it very quickly. And that is when I said to him, 'Sir. Please. Speak in English, please.'"
According to ABC, reports of the Facebook post first circulated on Tuesday morning, and four Facebook accounts have been linked to Mateen.
The official radio station for ISIS, Al-Bayan, said Monday morning, "Over 100 Crusaders killed and injured after an attack on their gathering at a night club in America ... We begin from America, where Allah has enabled brother Omar Mateen, one of the soldiers of the Caliphate in America, to carry out a raid where he was able to infiltrate a Crusaders' gathering at a gay nightclub in Orlando, Florida. Allah enabled him to inflict heavy casualties amongst the filthy Crusaders. He killed and injured over a hundred of them. This is the biggest raid to be carried in America after the raid of Manhattan 16 years ago. All praise to Allah."