WASHINGTON—A Senate investigation into the Central Intelligence Agency’s detention and interrogation of terrorist suspects after the Sept. 11, 2001, attacks concluded that its techniques weren’t effective, that CIA management of the program was deeply flawed, and that the program was “far more brutal” than the agency had acknowledged.

The report, which was written by Senate Democrats and wasn’t supported by Republicans on the panel, culminated an investigation that began more than five years ago and reignited an intense debate...