Federal records indicate that between 2007 and 2015, more than 1,500 US citizens spent time in immigration detention. And those are the just the numbers acknowledged by the government. Is your unlawful entry/reentry client really a US citizen? Read this article about citizens illegally detained by ICE, and get inspired to challenge the government's case for alienage.
Remember: Challenging alienage is not as complicated as trying to undo an underlying removal order. Alienage is an essential element that the government is required to prove. Demand exculpatory discovery regarding this essential element. Make the government prove this element at trial.