Earlier, this year, a Mexican think tank — the Citizens' Council for Public Security and Criminal Justice — released a study ranking the world's most violent cities in 2011, and the results were astonishing.

The 20 most violent cities were all in Latin America.

The USA had some alarming scores too, led by New Orleans at 21.

Other dangerous cities were located in Africa. None of the top 50 were in Europe and only one was in Asia, despite great unrest in those regions.

The ranking is based on murder rate per capita in 2011.