Brazilian officials are insisting security won't be a problem for the 2016 Olympics despite drug-gang violence that plunged Rio de Janeiro into a day of bloody chaos just two weeks after it was picked to host the games.

An hourslong firefight between rival gangs in one of the city's slums killed at least 12 people, injured six and saw a police helicopter shot down and eight buses set on fire Saturday.

Police said Sunday that they killed two other suspected drug traffickers in overnight clashes near the Morro dos Macacos ("Monkey Hill") slum where the gangs fought for territory a day earlier, but otherwise the area was largely peaceful.

Two officers died and four were injured Saturday when bullets from the gang battle ripped into their helicopter hovering overhead, forcing it into a fiery crash landing on a soccer field. Officials said they did not know if the gangs targeted the helicopter or it was hit by stray bullets.

Gunfire on the ground killed 10 suspected gunmen and wounded two bystanders.

Authorities said the violence would only toughen their resolve to improve security ahead of the Olympics and before 2014, when Brazil will host the World Cup soccer tournament with key games in Rio, the country's second-biggest city.

Rio state Gov. Sergio Cabral grimly told reporters Saturday that the city's security challenges can't be cured "by magic in the short term." But he said that money is being poured into programs to reduce crime and that authorities are prepared to mount an overwhelming security presence at the sporting events to ensure safety.

"We told the International Olympic Committee that this won't be an easy thing, and they know that," Cabral said. "We can put 40,000 people on the streets — federal, state and municipal police — and pull off the event."

Saturday's fighting raged about five miles southwest of one of the zones where Rio's 2016 Olympics will be held.

It was on Oct. 2 that the city was chosen over Chicago, Madrid and Tokyo to host the games. Rio alone among the bid cities was highlighted for questions about security ahead of the vote by the International Olympic Committee.

Rio is one of the world's most dangerous cities. Although violence is mostly contained within its sprawling shantytowns, it sometimes spills into posh beach neighborhoods and periodically shuts down a highway linking the international airport to tourist destinations.

Brazilian President Luiz Inacio Lula da Silva has played down the threat of violence for the Olympics, saying Rio has repeatedly demonstrated it can put on big events without risks to participants. The Pan-American Games in 2007 were held without major incidents after authorities deployed 15,000 specially trained officers.