After the official announcement this week that it was a bomb that destroyed Pan Am Flight 103 over Scotland, U.S. officials are vowing to find the culprits and tighten airport security.

But Americans hadn't better hold their breath waiting for the government to make good on either pledge.To begin with, it still isn't certain whether the blast was the work of terrorists or a criminal act of an angry or deranged individual.

If terrorists were involved, such guerrilla operatives are elusive and highly adept at finding new openings for attack when gaps in security are plugged.

At this point, the evidence seems to point in the direction of terrorists. Among the luggage abroad the Pan Am plane was debris that a top British laboratory has identified as containing residues of a powerful plastic explosive - a favorite terrorist weapon.

Terrorists use this explosive because it cannot be detected by the X-ray devices that screen passengers and luggage at airports. Though new screening devices capable of detecting the sophisticated explosive are being developed, it could be two years or more before they are available.

Even after the improved screening equipment is in use, it's high cost of as much as $1 million per machine seems bound to add substantially to the cost of an airplane ticket. If airports acquire only a few of the devices in order to curtail costs, the limited number of new screening machines seems bound to create long lines at air terminals and add to travel delays.

Moreover, terrorists could easily avoid the improved screening at airports just by switching to new weapons or moving to other, more vulnerable targets such as luxury liners or even buses carrying passengers to and from airports.

The message is clear: When it comes to airline safety, the U.S. must do whatever it can to protect the public. But the fact remains that, despite technological advances, security measures never will be perfect.