The group that conducted the research, the Institute for Studies on Religion (ISER), tallied the deaths of 10,216 people from 1995 to 2007 in police raids on the favelas (shantytowns) and other places inhabited by drug gangs.
It found that the ratio of dead to wounded climbed significantly after the city started offering bonuses to police in 1995 to particularly courageous officers.
In 1995, the ratio of killed to injured was 1.7 dead for every person injured.
That ratio has more than doubled, to its current 3.5 dead for every person injured. If police were not seeking to kill suspects, a greater proportion of injured might be expected, the report noted.
The man who wrote the report, sociologist Ignacio Cano, called the decision to pay bonuses a "Wild West incentive" which encourages police to kill suspects.
"It consolidated armed conflict as a security policy," he said.
The report was based on statistics from the Rio de Janeiro state government.
Despite the rampant violence, Rio de Janeiro was chosen last month to host the 2016 Olympics, the first edition of the Games ever to be held in South America.
Since then, scores of people have been killed in the city's drug wars. Police justify the deaths of most of these civilians, arguing that they are criminals who resisted arrest.
Rio's Public Security Ministry did not dispute the figures but insisted that the city "will not give up repressing drug traffickers" and do whatever it legally takes to quell the plague.
"Police have the obligation to act in that context," the ministry said in a statement.
Cano noted that the death toll of more than 10,000 for 12 years was worse "than many wars" and did not contribute to reducing violence in the city.
In comparison, the financial capital of Brazil, Sao Paulo, reported 40% fewer people killed by police in the same time period, even though it has 2.5 times more people than Rio do Janeiro.
Mercopress