Because of the city's proximity to sea level, it is difficult, if not impossible to have underground (basement) dwellings in New Orleans.
One hostage-taker out of three, Dova (Matt Dillon), poses as one of the hostages and escapes the situation along with two legitimate hostages, who earlier have vowed to remain silent about who was really involved. Despite the silence of the hostages, they and Dova would all be carefully identified by police before being released, and so Dova wouldn't be able to make a clean escape.