RIO DE JANEIRO (Reuters) - Rio Games organisers stepped up police patrols around the Olympic Park on Wednesday after a bus was hit by what security officials said were stones hurled by vandals.

Passengers on the bus reported hearing the sound of gun shots before two windows shattered as it passed just north of Olympic Park on Tuesday night. However, a preliminary police investigation concluded the projectiles were stones.

Two people suffered minor lacerations in the incident. Adding to concerns over Games security, organisers said they were investigating reports of a second bullet discovered at the equestrian centre.

A security source said the shell was discovered near the stables, used to house thoroughbreds for competition - some of the world's most expensive animals.

Police have said a stray bullet that hit the equestrian centre on Saturday was fired by a high-velocity rifle from a nearby slum.

Authorities say it was targeting a surveillance blimp being flown over the city during the Games. The incidents follow a series of robberies of athletes, journalists and spectators during the first Games to be held in South America, which started on Friday.

Luiz Fernando Correa, security chief for the Games, said stones had hit the metal rim of the bus' windows, making a loud noise, before smashing the glass.

"We have not been able to identify who did this but it appears to be an act of vandalism, not an act of criminal aggression targeting anyone in particular," he told reporters.

He did not specify if they had found the stones. Correa said police would step up patrols along roads linking the Olympic Park with the Deodoro stadium where basketball events are being held. The route passes several poor neighbourhoods, and only Olympic buses and military personnel were travelling along it on Wednesday morning.

A police source told Reuters it was the third time an Olympics bus had been targeted by youths throwing stones.

"This was a worrying and intolerable incident," Rio 2016 spokesman Mario Andrada told a news conference, adding that organisers were striving to improve security.

Andrada said a separate incident, involving a man who impaled himself after falling off a fence in the media complex, was a "one off" and had nothing to do with the quality of facilities. The man was rushed to hospital.

In relation to the bus attack, one of the passengers said she was unsatisfied with authorities' explanations. Sherryl "Lee" Michaelson, who was travelling on the bus when it was hit, said she had heard shots just before the windows shattered.

She said she was a retired U.S. air force captain and is working in Rio for a basketball publication. A Reuters photograph taken in the first moments after the bus was attacked showed a small hole, about the width of a finger, in one of the windows.

"I will not believe that was stone-throwing unless I see a forensics and a ballistics report looking not at the steel surround... but at the glass, which was the point of impact," Michaelson told reporters after the news conference.