PORT-AU-PRINCE, Haiti—After he climbed the bloodstained staircase, Carl Henry Destin found a baffling scene.

The Haitian president lay dead on the floor, with multiple gunshot wounds. Every drawer was flung open, and papers were scattered as if someone had been searching for something.

“The bedroom had been totally ransacked…documents everywhere,” Mr. Destin said. “There were a lot of witnesses, but they didn’t want to talk.”

Mr. Destin, a judicial officer often tasked with logging evidence at a murder scene, counted dozens of bullet holes and their locations at the presidential residence. He was struck by the chaos of the scene and the thin recollections from the bystanders who described little more than hearing the clatter of gunfire.

Outside, police frantically halted traffic as they searched for Colombian mercenaries they said had been running through the narrow streets of the hillside neighborhood.