Two weeks ago, President Bush signed into law a long-awaited Aviation Security Bill.

The bill had been stalled in Congress but, under a compromise reached between congressional negotiators and the White House, as many as 28,000 workers screening passengers and baggage will become federal employees within a year.

Five airports are to participate in a trial programme in which private contractors will provide security. Within three years, airports will have the option to decide whether they want to continue using federal employees or switch to private screeners.

In the meantime, extra National Guard troops have been deployed at airports to assist with surveillance.

The Aviation Security Bill also calls for stronger cockpit doors on planes and an increased presence of armed federal marshals on flights.

Despite the heightened sense of awareness, there have been security lapses. Four weeks ago, at Chicago's O'Hare International Airport, a man got past the security checkpoint with several knives and a stun gun. What helped to detect him was a computer programme called Computer Assisted Passenger Screening (CAPS) that all the major airlines in the United States have had in place since 1998.

By using this passenger profiling system, the airline computers flagged the man as a security threat. This prompted airline employees to search his bags at the departure gate where they found his weapons.

The Federal Aviation Administration (FAA) developed CAPS, which plugs into the US airlines' reservation systems.

This flags passengers, using certain criteria such as those passengers who pay with cash shortly before the plane's departure, buy one-way tickets or have an unusual travel history. The system does not factor in a person's ethnicity.

Interestingly, the system did not flag any of the 19 hijackers of September 11th, many of whom had bought first-class, one-way tickets.

Before the plane hijackings, passengers who were flagged had their checked baggage screened by trace detection equipment for explosives.

Now, a passenger's carry-on luggage, which is limited to one per person, is also hand-searched and he may be questioned and detained.

The Aviation Security Bill requires all baggage, mail and cargo carried in an aircraft to be examined by bomb-screening machines. The CAPS programme must be used to screen all passengers, not just those who check in at the ticket counter.

The FBI and law-enforcement agencies have given airlines access to their watch lists of suspects. This means if an airline agent is booking a reservation for someone whose name is already flagged on the computer, then the programme will not issue him with a boarding pass.

Mr Paul Takemoto, a spokesman for the FAA in Washington DC, said all the major US airlines are under instruction to use the explosive detection system continuously.

Since September 11th, "we have greatly increased the number of bags scanned", he added.

The CAPS system can also match bags to passengers. Bag-matching is a security measure in which a passenger's bags may not be transported unless the passenger is on the flight. It is already done for travellers on international flights and has been done on a limited basis for domestic flights.

CAPS uses information from the reservation system to screen out passengers who do not need the additional security procedures. If not enough information is known about a passenger to make a judgment, then the additional security measures in the form of explosives detection device screening or bag-matching are applied. CAPS will also select some passengers at random for these additional security measures.

There is no doubt that screening in the United States was lax before the terrorist attacks.

More often than not, you only needed to show one form of identification at check-in. That ID could be a credit card with an attached photograph, a driver's licence or a passport.

You could even book and pay for your ticket online and then swipe your credit card at a kiosk in the airport to check in.

The only time you were asked for some form of ID was at the departure gate before you boarded the plane.

After the attacks, kerbside check-in was banned at all airports, even though 40-60 per cent of passengers normally use it. The FAA has since lifted the ban, although some airport authorities have still not resumed it.

Those that do allow such check-in procedures must ensure that passengers are checked against the airlines' computers using CAPS. If the passenger is flagged, then he has to go into the terminal ticket counter and check in as usual.

Some people, especially those who lost loved ones in the Pan Am 103 terrorist attack, when a bomb was hidden in a piece of checked luggage that exploded in mid-air, believe all bags - checked and carry-on - should be screened.

However, the US airline industry does not advocate either bag-matching or 100 per cent checked bag-screening, claiming it would slow down air travel and would not have stopped the September 11th terrorist attacks.