Just after midnight on Oct. 28, a group of police officers were startled by roughly 20 armed men on motorcycles fleeing the Alemao favela, one of the headquarters of Rio de Janeiro¡¯s most notorious gangs.

A shootout broke out. Two of the men were hit and fell. As they lay dying from their wounds, they told the officers the group had ¡°leaked information¡± that police were closing in on the headquarters of the Comando Vermelho gang, and they were looking to escape, according to a police incident report from that day.

A couple of hours later, fireworks rang out over Alemao and the neighboring Penha favela, a common way of warning that police were approaching, two residents said. People started taking shelter wherever they could.