The Oakland Athletics said in a statement posted to Twitter that the incident happened after 9:30 p.m.
“Following last night’s Oakland A’s postgame fireworks show, officers were notified of a possible shooting victim inside the ballpark,” the team said.
“During the preliminary investigation, officers located three fans struck by bullet fragments from what appears to be celebratory gunfire occurring throughout the city of Oakland.”
The statement said a fourth victim later walked into a local hospital.
All of the victims were reported to have non-life-threatening injuries.
The Oakland Police Department (OPD) and Crime Stoppers are offering a reward for information leading to an arrest, the statement said.
“OPD will continue working closely with the Oakland A’s security to ensure the safety of all those attending future A’s games at the Coliseum,” the team said.
The Blue Jays face the A’s in Oakland again Tuesday night.