The FBI is investigating an incident in which a Border Patrol agent shot at one of several people standing on a border fence near San Luis, Ariz., and throwing rocks at him.

FBI spokesman Manuel Johnson says he couldn't confirm whether anyone was struck.

Border Patrol spokesman Ben Vik says the agent was patrolling between parallel fences Tuesday when two men dropped a ladder they were carrying and climbed back over the south fence into Mexico.

Vik says the agent, attacked while retrieving the ladder but uninjured, fired his M-4 rifle once.

According to Yuma's The Sun, a Mexican newspaper said doctors removed a .40-caliber bullet from a 23-year-old man who had been shot once in the back.

The agent's rifle fires a .223-caliber bullet.