A small plane entered restricted airspace Saturday night around Camp David, where President Bush was spending the weekend, prompting federal officials to dispatch a fighter jet to intercept it and track it until it landed at an airport in Frederick.

Authorities questioned the pilot of the Cessna 172 and released him. The aircraft was among three small planes that violated the restricted airspace Saturday. The breaches illustrate how increasingly complicated it is for civilian pilots to traverse the most controlled airspace in the nation -- and for federal officials to protect it.

In fact, such incidents are becoming so routine that the vast majority go unreported. Air defense officials said that since Sept. 11, 2001, they have sent fighter jets to respond to more than 2,000 violations of airspace across the United States.

Inside the so-called Air Defense Identification Zone, a 2,000-square-mile area of protected airspace around Washington, there are an average of two to three airspace violations a day when it's clear enough to fly small aircraft, said Laura Brown, a spokeswoman for the Federal Aviation Administration.

There were previous times when fighter jets intercepted small planes flying into the protected airspace around the Camp David retreat, said Lt. Jody Vazquez of the North American Aerospace Defense Command.

"This is not an uncommon thing to happen," Vazquez said. "We've had more than 2,000 threats, but a 9/11 has not occurred again. We consider this a success."

The complexity of the system continues to trigger the capital's alarm systems. On Wednesday, the Capitol was evacuated after a small, fast-flying plane breached Washington's restricted airspace. The intrusion disrupted a Senate vote and prompted Bush to be rushed to a safer location.

And on May 11, another small plane violated restricted airspace, prompting lawmakers and government workers to flee the Capitol, the White House and other buildings.

It was unknown yesterday whether Bush was moved from Camp David after Saturday night's incident. The White House referred calls to the Secret Service, which would not comment.

On most days, the small planes would not have been in violation. They were all flying well outside the three-nautical-mile no-fly zone that rings Camp David, which sits near Thurmont, north of Frederick.

But this past weekend, with Bush there, the radius of the restricted airspace was temporarily widened to 10 miles, part of a roster of post-Sept. 11 security measures.

"They weren't totally aware of the no-fly zone -- exactly where the borders were," said Cpl. K. Sherwood of the Maryland State Police, referring to the husband and wife who rented the plane, which was white with blue, maroon and yellow trim.

The plane bore at an altitude of 2,400 feet into the restricted airspace about 10:40 p.m. The couple apparently did not have the plane's radio on, said federal aviation officials, and had not filed a flight plan. Private planes require a distinct transponder code so authorities will know their identity, and pilots are required to maintain radio contact with the FAA as they approach restricted areas.

Air controllers watched the tiny blip on their radar screen as it moved deeper into the restricted airspace, but there was no way to contact the pilot, Vazquez said. So air defense personnel from NORAD dispatched an F-15 fighter jet to intercept and follow the Cessna. By then, it was 71/2 miles north of Camp David.

"We were just following the aircraft," Vazquez said. "The pilot was not responding to the radio."

Minutes later, the Cessna landed at Frederick Municipal Airport, where Secret Service agents were waiting to question the husband, the pilot. No charges were filed, and he was released, said Jonathan Cherry, an agency spokesman. Police yesterday would not identify the couple who had rented the plane, registered to Frederick-based Airworthy Aviation.

The first of the other two small planes that briefly breached the restricted airspace Saturday strayed inside the zone at 11:45 a.m. about eight miles south of Camp David and remained inside for about seven minutes, Brown said. It then flew out and landed in Lincoln Park, N.J.

The second plane skimmed the edge at 12:42 p.m. about 10 miles north of Camp David and left the restricted airspace after three minutes.

Both planes were tracked by radar, Brown said, and FAA investigators plan to question the violators. No fighter jets were dispatched in those incidents.

Jeff Myers, a spokesman with the Frederick-based Aircraft Owners and Pilots Association, which has more than 400,000 members nationwide, said such breaches by small planes "clearly" pose "no real threat" to the nation's security.

"The issue here is not the damage that can be done. It's the fear of what might happen," he said. "If somebody wanted to do huge harm to the D.C. area, there are so many other ways of doing it than small planes.

"It's great to be worried about a threat, but let's be worried about the right threats," he added. "No small airplane has ever been used in an act of terror anywhere in the world."

Still, Myers said, there was no excuse for pilots in the region to be violating the airspace zones around Washington. His organization, he said, is trying to educate its members to better plan their flights and get permission to even go near restricted zones.

"We're frustrated by pilots who aren't heeding the warnings," Myers said. "And those warnings are plentiful."