Evidence found in Boelter’s SUV made it “pretty clear” he was planning these attacks “for quite some time,” but his motivation is still unclear, according to acting U.S. Attorney Joe Thompson.

Vance Luther Boelter, 57, was arrested on Sunday for allegedly killing a Minnesota Democratic state lawmaker and her husband while posing as a police officer, Governor Tim Walz said.

Authorities say he also shot Sen. John Hoffman, a Democrat, and his wife, Yvette.