By Caroline Stauffer and Daniel Flynn

RIO DE JANEIRO (Reuters) - A stray bullet cut through the plastic roof of the equestrian media center on Saturday, alarming journalists and underlining concerns about the host city's ability to guarantee security at the world's biggest sporting event.

No one was injured in the incident. The equestrian venue is sited next to a military complex, though police are still trying to discover who fired the bullet and from where, a spokesman for the Rio 2016 organizing committee said.

"It was a stray bullet and has nothing to do with the Games," Mario Andrada said at a news conference, adding that security around the venue had been increased as a precaution.

Defence Minister Raul Jungmann was in the area accompanying the investigation and would return there on Sunday, a spokesman said.

Dozens of police and troops patrolled the streets en route to the equestrian complex. The area was not evacuated.

Some of the world's most expensive animals are housed nearby, and the dressage phase of the eventing competition was being held just a few meters away.

The bullet struck around 1 p.m. local (1600 GMT), when the press room was crowded at the end of a lunch break.

A spokesman for New Zealand's Olympic team said the bullet had fallen close to where the team's press attache was seated.

"She was sitting down and it went between the door and ceiling above her...," said Rob Waddell, New Zealand chef de mission and an Olympic rowing gold medalist.

He said New Zealand police liaison had advised the team that they were not being targeted, though the incident was still being investigated. New Zealand team members have been advised to stay inside competition venues, he said.

"We continue to assess the situation and look at the security around the venues and also in the village," Waddell said, adding that renowned horseman Mark Todd, who holds five Olympic medals, competed roughly an hour after the incident.

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

Also on Saturday a bomb squad detonated an unattended backpack near the end of the cycling course.

Killings in Rio and nearby Baixada Fluminense climbed 7.5 percent to 1,518 murders in the first six months of this year compared to the same period in 2015.

(Reporting by Caroline Stauffer; Editing by Daniel Flynn, Mark Bendeich and Peter Rutherford)