A white gunman shot dead nine people in an African-American church in the city of Charleston on 17 June, police say, two months after an unarmed black man was killed by a white police officer in North Charleston.

The killings have reignited the debate about race relations in the US. Here are six surprising statistics that help tell the story.

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There are only two black billionaires in the US - chat show host Oprah Winfrey and former basketball star Michael Jordan. This compared with about 500 white billionaires.

9.8% over-25s have a degree

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Black students appear to do well in high school - just over 30% of over-25s have a diploma, compared with 27% of white over-25s. But fewer than 10% of black over-25s completed a bachelor's degree, compared with 14.4% of whites.

75% of white Americans have only white friends

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A survey conducted by the Public Religion Research Institute in 2013 asked a range of questions to assess how diverse people's friendship groups were. It found that while 65% of black Americans reported only having black friends, 75% of white Americans said they had only whites in their social circle.

10.9% black unemployment

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Unemployment remains far higher among the black population, than the white. Currently 4.8% of white Americans are out of work, compared with an overall average of 5.8%

37% male prison inmates are black

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Figures based on the prison population at the end of December 2013 show black Americans made up a disproportionate number of inmates. Some 32% were white and 22% Hispanic.

Data sources: US Census, Public Religion Research Institute, Forbes, US Dept of Labor, US Dept of Justice

Correction: An earlier version of this story did not make clear that the figures for people with high school diplomas or bachelor's degrees were for those aged over 25.