Rio 2016 Olympics - Police have discovered a second bullet inside the Olympic equestrian centre close to the stable area, a security source said.
It is the second time a stray shot has been picked up at the venue over the past week.
No-one was injured and police were investigating to determine where the latest shell had come from.
The international equestrian federation said the dressage competition was continuing as normal.
The head of Rio 2016 communications Mario Andrada said he was in the stables "trying to find out what really happened".
On Saturday, Games off to shaky star a stray bullet tore threw the roof of the equestrian press centre, passing over the head of the New Zealand press attache and landing at the feet of a photographer.
The defense minister said it may have been fired by a gang member trying to shoot down a police blimp.
Brazilian police use blimps carrying security cameras to monitor crowds.
The blimp was flying on Tuesday during the final eventing competition.