The New York Stock Exchange’s internal regulators are investigating the decisions of a market-maker who operates on the trading floor after several investors said he caused them losses on October 3, according to people familiar with the matter.

Market-makers are supposed to help stocks trade in an orderly fashion, especially during special events when volume may increase several fold. But in this case, several firms trading the stock told NYSE regulatory staff the market-maker made things worse and cost them money, the people said.