The man came to the ticket window at Del Mar Horse Racing and wanted a ticket to the evening's Ice Cube concert.
When he was told no more tickets were available, he started an argument, pulled out a silver-plated semi-automatic handgun and fired several shots into a crowded area, according to the report of the San Diego Sheriff's Department.
Deputies on hand immediately responded by returning fire.
The gunman was taken to hospital; although his current condition hasn't officially been announced, there are unconfirmed reports that he died.
Ambulances and helicopters were also seen shortly after the shooting.
Simon Bray, a racing commentator, confirmed an active shooting in a tweet.
Visitors have also shared their stories about the shooting.
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There have been no reports of anyone aside from the would-be assailant being injured in the incident. The police have launched an investigation into the shooting.