EL PASO — Arlinda Valencia was at a funeral when an uncle told her a bewildering family secret: An Anglo lynch mob had killed her great-grandfather.

“A mixture of grief and shock overwhelmed me since this was the first I heard of this,” said Ms. Valencia, 66, the leader of a teachers’ union in El Paso. “The more I looked into it, the more stunned I was at how many Mexicans were lynched in this country.”

Ms. Valencia and other descendants of lynching victims are now casting attention on one of the grimmest campaigns of racist terror in the American West: the lynching of thousands of men, women and children of Mexican descent from the mid-19th century until well into the 20th century.

Some victims were burned alive, like Antonio Rodríguez, 20, a migrant worker who was hauled from a jail in Rocksprings, Tex., tied to a tree and set ablaze in 1910. Other mobs hanged, whipped or shot Mexicans, many of whom were United States citizens, sometimes drawing crowds in the thousands.