By Ossian Shine and Caroline Stauffer

RIO DE JANEIRO (Reuters) - Brazil's government is investigating whether a bullet that tore through the roof of the press center in the Olympic equestrian center was fired by a gang member trying to shoot down a police blimp or drone, the defense minister said on Sunday.

The bullet ripped through the plastic roof of the venue's media center on Saturday, missing journalists by just a few feet. It prompted the sport's governing body to demand an explanation and a security guarantee.

"One hypothesis is that someone who felt they were being filmed and observed made the shot," Defence Minister Raul Jungmann told reporters on Sunday.

Despite the unusual incident the venue was not evacuated and a second day of dressage tests in the three-day eventing competition wrapped up calmly.

"Obviously this is a worrying situation and is not an incident we can take lightly as the safety of everyone at our venue – athletes, horses, media and spectators – is of prime importance," Ingmar De Vos, president of the International Equestrian Federation (FEI), said.

Security has been increased at the venue since the incident, with a dozen soldiers seen guarding the entrance and more patrolling nearby streets.

"The FEI is actively monitoring the situation and we have asked for confirmation about the measures that have been put in place to ensure there will not be a recurrence," Vos said.

Security has become a major concern for organizers of South America's first Olympics. Brazil has deployed 85,000 police, military, and private security personnel for the Games, more than double that of London in 2012.

The equestrian venue is sited next to a military complex, fuelling speculation that the bullet could have come from a firing range.

Jungmann said the bullet absolutely did not come from the Olympic shooting competition held at a venue more than 2 kilometers (1.2 miles) away.

'THERE'S A BULLET ON THE FLOOR'

With high levels of violent crime in Brazil's slums, some of which are controlled by drug trafficking gangs, stray bullets are a concern for Rio residents. Drug gangs in the favelas or slums are fiercely protective of the privacy of their operations from police and rival gangs.

A Rio 2016 organizing committee spokesman said on Sunday that Brazilian police commonly used blimps and drones to monitor crowds.

"The blimps started to fly just a bit before the Games," Mario Andrada told a news conference. He insisted, however, that the Games themselves were not being targeted.

Saturday's bullet, bronze in color, landed close to American photographer Rob Carr.

"We were sitting down there a little after one o'clock. Myself and my colleague were sitting about 10 feet up and you heard this cracking and rattling sound that sounded like something hitting metal and like five seconds go by and someone's like 'Oh my God there's a bullet on the floor'," he told Reuters.

"That bullet was 10 feet away from me and closer to a lot of people."

Carr said he had been listening to gunfire all Sunday morning.

A military spokeswoman said there were no drills at the neighboring facility. Two soldiers stationed outside the equestrian center said any shots heard must have been from the Olympic shooting venue.

(Additional reporting by Anthony Boadle in Brasilia and Rodrigo Viga Gaier in Rio de Janeiro; Editing by David Evans and Andrew Hay)