A self-described debt collector for Mexican drug cartels says he has slayed more than 30 people across the United States, according to investigators. If Jose Martinez, 51, is found guilty of scores of homicides on both coasts, he would earn a place among the most lethal serial murderers in American history. Investigators have confirmed that Martinez is responsible for a 2006 double-homicide in Marion County, Fla., a March homicide in Lawrence County, Ala., and at least 10 other killings in California, according to sheriff’s officials. Martinez, a U.S. citizen, told investigators that he committed those murders and more than a...