Rio 2016 organisers said two journalists suffered slight injuries in the attack, which set off a new Olympic jitters four days after a bullet hit a Games media centre.
Journalists in the bus hurled themselves to the floor after two windows were blown out. The driver put his foot down and got the passengers to the Games’ main press centre.
Rio 2016 said in a statement that police and army patrols had been stepped up but gave few indications on the cause. Rio is notorious for gun crime and gang violence.
The bus was returning from the Deodoro Olympic zone to the main press centre and was passing near the City of God slum district when it was hit.
“Two impacts on the right side of the bus shattered two windows. A reporter from Belarus suffered a cut hand. There is confusion as to whether it was bullets or stones,” Gaston Sainz of the Argentinian daily La Nacion, who was on the bus, told AFP.
“We threw ourselves to the floor and two kilometres later the police came and escorted us to the main press centre with the broken windows,” Sainz said.
According to the organisers’ statement, “the driver heard a noise from inside the bus which he thought was photography equipment falling down.
“Immediately, he looked in the rear view mirror and noticed that the passengers were lying on the ground.
“He continued to drive for a few metres until he saw a police car and stopped. At this time, he realised that two windows on the same side of the bus were broken.”
The driver went on, escorted by the police car, “and the broken windows began to give way further”.
On arriving at the media centre two passengers had minor injuries caused by broken glass cuts, the statement said.
The driver was to give a statement, while the bus was being examined by police.
Security forces had started “increased patrols”, the statement said.
The International Olympic Committee said it would wait for a police report before commenting.
But it was the latest in a worrying series of incidents to unnerve Games participants.
A stray bullet flew through the roof of a temporary media work room at Deodoro on Saturday and landed near a New Zealand official.
Defence Minister Raul Jungmann said the bullet could have been aimed at a drone or balloon flying over the district with a security camera.
The same day, security forces conducted a controlled explosion near the finish line of the men’s cycle road race.
Another suspect package was blown up on Tuesday near the luxury beachfront Copacabana Palace hotel.