reporter: bruna silva is a broken woman. r son marcos was murdered on his way to school he in mare,, a favela in rio de janeiro. bruna: it was here that he died. when marcos turned the corner on his bike, he saw the military police. he wanted to turn back, but then he was hit in his side by a bullet, which tore straight through him. it was a high caliber weapon. reporter: he died under this overpass on the edge of the favela. the sprawling slum is home to more than 130,000 people. shortly after 8:00 in th mornin p police oop p on suspected d drug dealersrs in s densely y populated neneighbor. shots are fired from the air and from armed vehicles on the street. the target of the assault -- the criminal gangs that tyrannize mare. but as usual, the general public, including children, were caught up in the violence. >> there was a hail of bullets. there was a helicopter overhead. i had to hide under a bus. reporter: bruna shows us the bullet holes left in the street by machinegun fire from a police helicopter. the bullet holes are