Today, what we know about the four Americans who were kidnapped in Mexico, and what this incident can tell us about medical tourism, the security situation at the U.S. southern border, and how U.S. policy has contributed to the problems.Read more:Last week, four American friends from South Carolina were kidnapped in the Mexican border city Matamoros. By the time Mexican security forces located them on the outskirts of the city Tuesday, two of the Americans were dead and another was injured. The two survivors have been returned to the border, and one suspect is in custody with an ongoing manhunt for others. Today, The Washington Post’s Kevin Sieff explains how this affects the security relationship between the United States and Mexico, and what role the U.S. has played in making Matamoros a place where violence and kidnappings happen, often with impunity.