ALBUQUERQUE, N.M. (AP) — Twenty years ago, a knock on the door opened the past for Arlinda Valencia.

A relative had come to pay his respects on the death of Valencia's father. He then revealed a shocking secret: The family was descended from survivors of a 1918 massacre along the U.S.-Mexico border.

In an account later confirmed by Valencia's 96-year-old great-uncle, the Texas Rangers and U.S. soldiers killed her great-grandfather and 14 other men and boys. The massacre that all but wiped the town of Porvenir, Texas, was part of a campaign of terror that largely targeted Mexican Americans.

"Bu...