ACAPULCO, Mexico – Four gunmen abducted three U.S. citizens on a highway in southern Mexico, shot and killed two of them and left the third – a pregnant woman – bound and gagged, police said Saturday.

Reynaldo Valdez, a 22-year-old from Houston, was traveling with Miami natives Ashley Diniger, 16, and Vanessa Burgos, 22, to visit his mother, police said.

The three had left Cutzamala de Pinzon late Thursday when a vehicle carrying four men forced their vehicle off the road near Tierra Caliente, police said.

The gunmen drove them to a nearby town and robbed them. The abductors later tried to sexually assault the two women, then shot and killed Valdez and Diniger, police said.

They spared the life of Burgos, apparently because she was pregnant, but tied up her hands and feet and dumped her on the side of a road, police said. Her testimony led to arrests Friday of four suspects.