AUSTIN, Tex. — On Aug. 1, 1966, Charles Whitman ascended the University of Texas clock tower here with a trunk full of weapons and unleashed 96 minutes of terror that effectively became a template for mass shootings and aroused in the public a new sensitivity to the threat of violence in public spaces.

Mr. Whitman, a 25-year-old student, Eagle Scout and Marine veteran, killed a receptionist and two members of a visiting family inside the tower. He then went onto the observation deck and began spraying sniper fire, turning a tranquil summertime campus into a scene of chaos and death.

In the half-century since, Mr. Whitman’s savagery has been echoed in mass shootings on other university campuses and at workplaces, elementary schools, post offices, movie theaters and nightclubs. And what seemed unthinkable in 1966 was re-enacted with alarming repetition in places like Columbine High School in Colorado; Virginia Tech in Blacksburg, Va.; Sandy Hook Elementary School in Newtown, Conn.; and the Pulse nightclub in Orlando, Fla.

“When I hear there’s another tragic shooting at a university, my heart breaks and I relive every excruciating moment of pain and mental anguish, knowing what the survivors or loved ones of those who are injured are going to go through,” said Adrian Littlefield, a semiretired minister from Kirbyville, Tex., who was severely wounded in the University of Texas attack.