This is perhaps one of the most powerful images to become a polemic in the mainstream news media, both in Brazil and internationally. At a protest near the Governor’s Mansion, The Palacio Guarnabara, activists clashed with the riot squad during the Papal Visit in July. The violent confrontation started when someone threw a Molotov cocktail at the police line. The immediate response by the government and the media was to call those responsible murderers and terrorists. As it turns out, however, evidence strongly suggests that the petrol bomb was thrown by an undercover police officer. These infiltrating officers, known as P2, are supposed to mix amongst the masses for intelligence purposes only, but they have repeatedly been accused (and I say the word accused only because the judicial system refuses to investigate) of inciting violence in order to justify police repression.
The New York Times did an excellent job piecing together all of the citizen and journalist footage in a piece that summarizes how activists figured out that the true culprit was not protestor whom the police were charging with attempted murder, but actually an undercover officer: http://thelede.blogs.nytimes.com/2013/07/24/video-of-clashes-in-brazil-appears-to-show-police-infiltrators-among-the-protesters/?_r=0
[photo was found on Black Bloc RJ facebook page, however photo credit is unknown]