People working at the hostel tipped off the police that they recalled seeing someone staying there who always was wearing the same black face mask as the shooter. Police then went through the video from the hostel and found what looks like a match. The guy in the picture smiling apparently used a fake ID when registering at the hostel. Police also recovered a cell phone dropped at the scene. It was a burner phone.
This guy now has a three day head start on law enforcement. The only leads law enforcement have right now are a picture of a very average looking white guy who bought a gun and probably had a bad experience with United Heath Care. So, they have narrowed it down to about 60 million Americans.
If this guy was really good at this, he would have immediately taken a flight out of the US to a non-extradition country and live out his days trading bit coin on a beach villa in Nicaragua.