The man was picked up over the weekend, and is no longer a suspect, or even a "person of interest." He had returned his rifle to the dealer he bought it from (before the shootings, I believe), and no weapons were found in his possession. He is being held on a car theft charge from Florida. He was reported missing by his family on Monday, well before the shootings took place.
This was a case of the Maryland police grasping at straws for the perfect suspect: a white supremicist.