A patron reported to the station agent that a Hispanic male, unknown age, slim, wearing a white jacket, and black or blue jeans, had possibly stolen a phone from a female victim on platform #2. As this was happening, a witness, who attempted to intervene and come to the aid of the victim, was challenged by the suspect who brandished a kitchen knife. The witness backed off and followed the suspect out of the station. The suspect returned to the station as the witness followed. The witness flagged down two officers who eventually detained the suspect inside the station. The original victim of the theft could not be located and officers were unable to determine if there had been a theft. However, the witness, positively identified the suspect as the one who had brandished a knife. The witness declined to press charges against the suspect for the brandishing. The suspect was placed under arrest for possession of a dirk or dagger, and transported to the Martinez jail where he was booked for the offense. TSOC was notified.