The mob actions were born of a sense of hopelessness and impotence shared by many in Mexico, where 98 percent of murders go unsolved and the state is virtually absent in some areas. By some estimates, just 12 percent of crimes are even reported in Mexico, largely because of a lack of faith that justice will ever be served.

Such a void, taken to extremes, has found its resolution in violence.

“There is a crisis in terms of the growth of violence and crime and a parallel erosion of authority and the rule of law,” Mr. Guillén said. “These lynchings acquire a double meaning. People lynch both the suspect and the symbol of authority.”

Interviews with dozens of residents about the lynching of the brothers — David and José Abraham Copado Molina — revealed little remorse. In the end, the fear that two suspects might be escaping with the help of the police outweighed concerns over spilling innocent blood.