A lot of the zombie shrieking was added in at post-production stage. To shoot with numerous actors shrieking constantly would have meant that all the dialog would have been drowned out. Adding it in afterward meant that the sound technicians could manipulate the noise to its best advantage.
The SWAT team, police and crowd of onlookers outside the building were real people, not actors.
The constant presence of helicopters buzzing overhead was created entirely by the sound team. The production couldn't afford any real helicopters.
The actors playing the Special Forces team were given a day's intensive training in how to handle firearms.