CENTENNIAL, Colo. (CBS4) – A warning is now in effect and general aviation is on alert as Sept. 11 draws closer.

One fear is that small planes could be loaded with explosives and flown into a stadium filled with sports fans.

READ MORE: Early Look At Weather This Halloween Shows A Weak Storm Could Be Near Colorado

The intelligence bulletin from the FBI and Department of Homeland Security reads, “Violent extremists with knowledge of general aviation and access to small planes pose a significant threat to the homeland.”

The planes are small, the targets big, and Al-Qaeda’s interest in them as a weapon is longstanding. Just last year an American man angry with the IRS flew a general aviation aircraft into a building in Austin, Texas.

An even greater fear now is a terrorist could pack a plane with explosives or chemicals and crash it into a stadium filled with thousands of people.

“It would not be an easy thing. It’s not a Ryder truck,” said Robert Olislagers, Centennial Airport Executive Director.

READ MORE: Remains Of West Coast Bomb Cyclone Will Bring Stormy Weather To Colorado On Tuesday

Olislagers said they have a layered system of security in place not readily available to the eye.

“We’ve known that there’s always been an interest on the part of the terrorists to use small aircraft for a number of purposes, including using aircraft as a missile,” he said.

Centennial Airport employees and pilots are paid gift certificates or cash if they provide a verified report of any suspicious activity.

Jeff Price is an aviation instructor at Metropolitan State College of Denver and author of a book on aviation security.

“The small aircraft with even a small amount of explosives on board, or even no explosives on board, crashing into a stadium, an open-air stadium like we have here at Mile High; that’s going to cause some casualties,” Price said.

While there is no indication of a specific threat as Sept. 11 nears, eyes will be on the skies.

MORE NEWS: Ryan Yarwood Arrested After Eluding Police In Stolen Vehicle

The Broncos will play in Denver the day after Sept. 11, starting their season Monday night against the Oakland Raiders.