BETHEL PARK, Pa. — Authorities said Monday that they had accessed the phone of the person who attempted to assassinate former President Donald Trump and had completed searches of his car and his family’s home in suburban Pittsburgh, as ongoing efforts to understand the shooter’s motives have so far come up empty.

The FBI said its investigation remains in the “early stages,” but the immediate lack of a clear motive for Saturday’s shooting at a Trump rally in Butler, Pennsylvania,…