The FBI identified the shooter, who is dead, as Thomas Matthew Crooks of Bethel Park, Pennsylvania. A motive was not clear.
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Bethel Park is a predominantly white, relatively well-to-do city in the southern reaches of greater Pittsburgh. The site of the rally, Butler, is about an hour’s drive north of Pittsburgh.
Pennsylvania voter records listed a Thomas Matthew Crooks with the same address and birth date as a registered Republican, though it was not clear from the records when that was put in place.
This is the right answer. It is entirely possible and feasible to register as a party you don’t agree with for strategic reasons such as voting in primaries for important state offices etc. Millions of people do this across various states.