RB_CH03_Q018. 20 traders 100 stocks SEC announcement

There are 20 traders in a room. They trade for their own accounts and only amongst themselves—it is a 'closed economy.' Halfway through their morning trading session, a group of SEC officials arrives and announces that one of the traders has made inside information on one stock and has been trading on it. The trader is not yet identified. The SEC officials seat themselves in the room to watch. What happens to trading volume after the SEC announcement? Explain carefully.