We know from what people posted after talking to the SEC about pirate that the SEC was asking questions about MtGox.  We also know that the email sent by the SEC said that information given to them could be shared with other agencies (which is pretty standard if there's a more appropriate agency to follow something up). 

It's highly possible that there were multiple different things which put MtGox on the government radar, but it's unlikely that a recent complaint by a competitor would have resulted in such quick action.