The film was shot during the hours between 1 and 5 AM.
The film was shot on two days a week over the course of 4 months. It took 26 days total to shoot.
Cast and crew would wait until the streets were clear of moving cars so as to give the feeling of desolation, despite being in the middle of one of the largest cities in the US.
The tunnels that the final investigation sequence was shot at is below a major university in Dallas. One of the key scenes was shot directly underneath the Olympic size swimming pool. The temperatures approached 100 degrees.
Actual homeless would wander by and hold up the shoot.