question: According to Castro, Which two criminal elements work together, even when in prison? answer: Drug cartels and police paragraph: (CNN) -- Three human heads and three decapitated bodies with notes aimed at high government officials were found Thursday in different parts of Guatemala's capital, national police said. The notes were addressed to Interior Minister Carlos Menocal and Eddy Morales, the nation's director of prisons. One of the notes said the officials must impose order in the nation's prisons or these atrocities would continue, police said. The other said that impunity against lawlessness must end, according to police. The heads were inside plastic bags, one of them in front of the main doors to the national Congress in Guatemala City, police said. "That's obviously a first for Guatemala," said Samuel Logan, an expert on Latin American gangs and founding editor of the Southern Pulse intelligence report. "That's something we've seen the Zetas (drug cartel) do in Mexico." Donald Gonzalez, a spokesman for the national police, attributed the slayings to the Zetas or another narcotrafficking group, the Prensa Libre newspaper reported. Gonzalez did not immediately return a phone call seeking comment. Logan did not discount the possibility that anti-crime vigilantes could have been responsible. "It could also be death squads," he said. "Off-duty police officers or other types hired by local businessmen who say, 'Go after the street gangs.' "There's a lot of moving parts to this story," said Logan, who recently wrote a book on the Mara Salvatrucha gang of Central America and is working on a book about the Zetas, which has emerged as Mexico's most ruthless drug cartel. Logan said in an interview last year that there are two main criminal elements in Guatemala: drug cartels that deal in large-scale exportation and street gangs that sell drugs at the retail level and are involved in other crimes, such as robbery and extortion. In some cases, they work together. When those criminals get arrested, they continue their activities inside prison, said Fernando Carrera Castro, director of the Central American Institute for Fiscal Studies in Guatemala City. 