I am not assuming he is or isn't using a phone or that he did or didn't even have one on him. I am saying that these are strong possibilities as cell phone analytics have caught many, many bad people.
His biggest mistake, and he clearly knows it now, is unmasking himself. Shortly after that picture was released, probably less than 30 minutes at most, one or more neighbors, one or more co-workers, a cousin who hated his guts and having to see him at Christmas, an ex-girlfriend (or boyfriend--who knows) had contacted the police as to his identify.
The police have a name and now a living address. Yes, he could have hopped around with at friend's houses or Family, but his location was found. Because of the act that he allegedly committed, there is a law that forces cell phone/internet providers to give up information...yes, there really is.
Most likely, he owns a cell phone and pays a bill, maybe not, but most likely. The cell phone company can provide his number and verify his last address. They can now trace that cell phone wherever he goes. Could he have thrown it out, of course. But not likely as he'd have no communication with whoever does or doesn't know about his actions whether it be prior to it happening or afterwards. Of course, no one knows that now except the police who can see who he called days and weeks prior to the shooting and who he called afterwards. On a scale of 1 to 10, it's a 2 a most in terms of difficulty connecting the dots.
Wherever he lived, he's long gone if he wasn't already caught as he knows they'll be able to find him through whoever turned him in within 30 minutes of when that photo was released. If he was on a bus, they would not have the bused pulled over as he'd take a hostage in a heartbeat and someone else would be killed by the nutjob. They'd observe it and wait as they know he's got a weapon.
There are several scenarios, but most likely he had a traceable phone and his name and address were known quickly after that photo of his mask down was shown. He's on the run and will be caught, or take his own life. Getting out for the country would be difficult as all airports would have his name on a watch list and he'd never make it on the plane.