Hundreds gather for a prayer vigil, with speakers urging peaceful demonstrations

The US Justice Department said on Wednesday it would investigate the killing of a black man pinned to the ground and shot in the chest by two white police officers outside a convenience store in Baton Rouge, Louisiana.

Graphic video images of Tuesday's shooting of Alton Sterling, 37, unleashed protests and social media outcry over the latest alleged police brutality against African-Americans in cities from Ferguson, Missouri, to Baltimore and New York.