Florida's MacDill Air Force Base — home to the command center running the war in Afghanistan — has air defenses in place in case someone tries a suicide attack, officials said Monday.

Those defenses were alerted Saturday when a teen-ager flew a small, single-engine plane across MacDill's airspace, but they took no action, the officials said.

"There are things in place that if we thought there was a threat, we could have done something," said Tech. Sgt. Chris Miller, a MacDill spokesman.

Charles Bishop, a 15-year-old student pilot, later flew the Cessna 172 into the side of a building in downtown Tampa, killing himself.

A defense official, speaking on the condition of anonymity, described the defenses as heavy machine guns manned round-the-clock. The water that surrounds three sides of the base is patrolled by Coast Guard vessels.

Those are probably sufficient to shoot down a small plane, said John Pike, a military expert with GlobalSecurity.org in Alexandria, Va. But they would have little effect on a hijacked airliner plummeting from the sky. The base doesn't have any surface-to-air missiles defending it, officials said.

Tampa's air traffic control center tracked Bishop's plane and informed MacDill when the plane was about three miles west of the base, Miller said. Bishop entered MacDill airspace and descended slightly, but left one minute later.

"We felt there wasn't any threat to MacDill," Miller said. Base officials were told the pilot's identity and concluded he probably wasn't a terrorist threat, he said.

An unarmed Coast Guard helicopter intercepted the plane and tried to get it to land, to no avail. The nearest military fighters on alert, two Air National Guard F-15s with the 125th Fighter Wing from Homestead Air Reserve Base near Miami, scrambled to intercept it, but they took off after the plane had crashed into the building, military officials said. After they arrived near the crash, they patrolled Tampa airspace.

Bishop was aloft between nine and 12 minutes before crashing into the 28th floor of the Bank of America building.

Gen. Tommy Franks, the Central Command chief at MacDill, said in an Associated Press interview Monday that he was not alarmed by Saturday's incident and saw no need for dramatic increases in base security.

"I think one would want to be careful before we decide that every potential place in our country that could be threatened should have its own combat air patrol and that sort of thing. So no, I wouldn't" advocate that, he said.

As they did on Saturday, fighters scrambled to intercept the hijacked airliners that flew into the World Trade Center and the Pentagon on Sept. 11, but they also arrived too late. Since Sept. 11, the North American Aerospace Defense Command — NORAD — has kept dozens more fighters on alert around the country, running constant patrols over Washington and New York, and random patrols over other key cities. About 30 air bases around the country have fighters armed and fueled, ready to be airborne in under 10 minutes, NORAD officials said.

Homestead was one of the few air bases that had fighters ready to scramble, even before Sept. 11. But its planes patrolled with an eye toward Cuba and drug smuggling flights off the Florida coast.

MacDill doesn't have any fighters of its own. The base instead houses U.S. Central Command, which has operational authority over the war in Afghanistan and other military operations throughout the Middle East. It's also home to U.S. Special Operations Command, which oversees the Green Berets and other commando units and aircraft that are conducting many of those operations in Afghanistan. MacDill's airfield also services a wing of midair refueling tankers.

Central Command has a secure operations center designed to function in adverse conditions, such as the hurricanes that batter Florida from time to time, said spokesman Lt. Col. Martin Compton.

The Pentagon is regarded as more of a potential terrorist target than MacDill. The Pentagon's internal security is at "Threat Condition Charlie," the second-highest state of alert. MacDill is at "Bravo," one step lower.