According to a letter written by Senate Committee on Homeland Security and Governmental Affairs Chairman Ron Johnson, Orlando shooter Omar Mateen posted to Facebook during the massacre, and he also checked the social media site to see if the attack was trending. Johnson revealed this information in a letter to Mark Zuckerberg, which he also made public. The letter asks for Facebook's cooperation in investigating the attack.
Apparently, in addition to posting things such as "America and Russia stop bombing the Islamic state..I pledge my alliance to abu bakr al Baghadadi..may Allah accept me," Mateen also used Facebook to search for the terms "Pulse Orlando" and "Shooting" while he was still inside the club. As a reminder, there were approximately three hours between when the shooting began and when Mateen was finally killed by officers.
The letter also notes that Mateen's last status posted to Facebook was "In the next few days you will see attacks from the Islamic state in the usa."
You can see the full letter text, as uploaded to Scribd by Mashable, below. And if you need a reminder that there is good in the world, read about the plane of strangers that took the time to comfort the grandmother of one of the Orlando victims.