What comes out within hours or days of most mass killings, such as the recent tragedies in Dayton and El Paso, is that the murderers showed signs of homicidal tendencies long before they acted.

Before police shot him to death Saturday, a 24-year-old man killed nine people and wounded 14 others in Dayton. His attack came as no surprise to some. Several former high school classmates said the killer was suspended from school for a time for compiling a “hit list.” And, they added, he also drew up a “rape list.”

“He enjoyed making people feel scared,” one former classmate said.

He succeeded. On the day in 2012 that news of the “hit list” surfaced, about one-third of the students at the high school stayed home, for fear of an attack.

Similar warnings came from the man who shot 22 people in El Paso to death. He had released a “manifesto” indicating hatred toward immigrants. Some former high school classmates described him as an “irritable loner,” according to one report.

But profiling people gets complicated. The Dayton killer favored left-wing politics. The El Paso murderer was just the opposite. Some people even remember the Dayton man as quiet and friendly — not threatening at all. The same contradictions can be found in the backgrounds of many mass murderers.

Until and unless we devote more energy — and realism — to finding out what makes a mass murderer, the killings will continue. And then we face an even thornier question: What can we do about potential homicidal maniacs? Seeming to be a threat is not likely to convince any courts that someone should lose his or her constitutional rights.