MYSTERY DEATHS THE PROBLEM: Guatemala is experiencing a wave of mysterious killings targeting gang members. WHAT'S BEHIND IT: Human rights groups claim vigilantes, possibly including rogue cops, are engaged in ''social cleansing.'' The country's attorney general blames gang warfare. THE BIG PICTURE: The Central American nation has endured 36 years of civil war and poverty is rampant. Gangs have been swelled by Guatemalans who immigrated to the U.S. and were deported back for gang activity. GUATEMALA CITY -- Some are shoved into cars with dark windows and never seen again. Sometimes bodies turn up with messages left on paper or...