The Case of the Unconquered Sisters (a Hugh Rennert Mystery)

Sisters Lucy and Monica Faudree live with their winsome niece Cornell in an elegant old classical revival house by the black lava fields of Mexico's Pedregal. Seven decades ago their grandfather, Tindall Faudree, a former Confederate military officer, forever abandoned the United States for Mexico ("Unconquered" is the family motto). Now his descendants are hosting an archaeological group from an American university, while it conducts an expedition in the Pedregral. Unfortunately another guest turns up uninvited at the Faudree mansion: Death!
Who sent the threatening letters with the bizarre drawings to the late, unlamented Professor Voice? Why did all those owls (tecolotes) descend on the Faudree house? Will there be another murder? You probably know the answer to that last question already! U. S. Customs Service agent (and amateur detective) Hugh Rennert again is on hand to show Death to the door.
When originally published in 1936, The Case of the Unconquered Sisters was praised in the Saturday Review for its "satisfactorily sinister blend of Mexican lore and scenery, swift action and . . . class A sleuthing." Can you beat Hugh Rennert to the solution? Don't let the hooting of the tecolotes trouble you. . . .