Georgetown, Texas ? After 9/11, cockpit doors were sealed, air marshals were added and airport searches became more aggressive, all to make sure an airliner could never again be used as a weapon. Yet little has been done to guard against attacks with smaller planes.

That point was driven home with chilling force on Thursday when a Texas man with a grudge against the IRS crashed his single-engine plane into an office building in a fiery suicide attack. One person inside the building was also killed.

“It’s a big gap,” said R. William Johnstone, an aviation security consultant and former staff member of the commission that investigated the 9/11 attacks. “It wouldn’t take much, even a minor incident involving two simultaneously attacking planes, to inflict enough damage to set off alarm bells and do some serious harm to the economy and national psyche.”

The suburban Georgetown Municipal Airport that pilot Joe Stack entered hours before his airborne attack in Austin had the casual atmosphere of a sleepy parking garage. Pilots are not subject to baggage checks, metal detector scans or pat-downs. And they’re usually not required to file flight plans.

“How are they going to stop it? This guy had a hangar, and he had access to the airport,” said Beth Ann Jenkins, president of Pilot’s Choice, a flight school near where Stack kept his Piper.

Travis McLain, manager of the airport, said: “I don’t know of a rule or regulation or safety precaution that could have prevented what happened yesterday.”

The easy access and lack of security are the result of years of debate — and stalemate — over how much of a threat small aircraft pose as terror weapons and how they could be regulated without stifling commerce and pilot freedom.

Private pilots fly about 200,000 small and medium-size planes in the U.S., using 19,000 airports. General aviation advocates argue that stringent security measures would deter recreational fliers and slow a vibrant, multibillion-dollar general aviation industry, causing economic damage.