One mistake that the old fogies in the media are making is the assertion that it's super unusual for a 20 year old not to have a presence on social media. It's actually becoming more and more common for kids to hide any traceable internet activity. Particularly if they are socially anxious, are being bullied, or feel like they need to hide from their parents or the authorities.
They choose to either avoid social media or more likely to use volatile social media or social media that they can control and keep from copying and leaving a permanent record. Discord is one of these platforms. I believe Snapchat and Instagram also can be used that way. Facebook is not which is why you don't see so many young people on there. Tumblr provides more pseudonymous anonymity at least from social groups that you may know in real life.
So if I had to sit down and think about it I'm pretty sure I would predict that there was a high probability that the alleged shooter would have no real traceable social media presence.