My sister called me freaking out because she found an American roach in her bathroom and now my nephew will not use the bathroom. American roaches can be intimidating and down right scary, but they don't always mean you have to panic right away. These large roaches are known in the city as sewer roaches, many times they appear when there is street work being done or pipes out int he street are being replaced. They can also appear in the home if there is a broken sewer line that you may be unaware of. In the latter case, these big roaches can be a blessing in disguise, alerting you to a much larger problem.
So having said all that, how do you get rid of them? Call in your local bug guy or gal and have a good residual put down, place some sticky monitors, and some granule baygon or niban bait. If the problem persists, call you plumber.