Unlike police, firemen and paramedics do not need warrants to get into homes and other buildings during technical inspections of emergency facilities, making them particularly useful for spotting signs of terrorist planning.
The Homeland Security Department has been secretly testing a pilot scheme in New York in which firefighters are trained to identify suspicious material or behaviour. If successful, the programme will be extended to other large cities.
Since the September 11 attacks, Americans have given up some of their privacy rights in an effort to prevent future strikes.
The government monitors telephone calls and emails, and there is much more thorough searching of belongings at airports.