CNN  — 

Police in suburban Atlanta say they’re trying to find a man who witnesses say pointed a rifle at a passing school bus this week and apparently left a note at the scene containing school bus numbers.

The incident in Hampton has prompted Clayton County police to start escorting school buses through the Greystone subdivision, and federal agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives have joined the investigation.

A resident of the subdivision told police that a man was crouched in a backyard of a home in Hampton – about 25 miles southeast of Atlanta – on Monday morning and pointed a rifle at a moving bus.

“About the time the school bus pulled up to pick up two kids … the guy started aiming the gun,” the resident, David Dillard, told CNN affiliate WSB.

Dillard said he yelled at the man, and the man dropped the rifle and ran away. Dillard said his nephew, who was nearby, ran after him.

The gunman fired a pistol at the nephew – hitting no one – before escaping on foot, Clayton County police Lt. Chris Windley said, citing witness accounts.

At the scene, police found a rifle and a note that listed numbers of school buses in Clayton County, Windley said. Police haven’t said whether the rifle was loaded.

Police said they and the ATF are investigating the incident, and they don’t have any suspects or know the gunman’s motive.

The ATF is trying to determine the rifle’s history and owner, bureau spokesman Richard Coes said.

Officers will follow school buses through the subdivision and patrol the area through Friday, the last day of the school year, unless an arrest is made before then, Windley said.

A resident of the area, Sean Rutherford, told WSB that his son was on the bus Monday morning.

“I (now) make sure I go pick him up from school instead of him riding the bus,” Rutherford said.

Windley said police are trying to determine whether they have enough of a description of the gunman to release a composite sketch. Douglas Hendrix, chief human resources officer at Clayton County Public Schools, said the gunman was described as a 18- to 23-year-old man wearing black clothing. The race was unclear, he said.

Schools in the area have “suspended all outdoor activities” for the students, Hendrix said.

CNN’s Meridith Edwards, Rick Martin, Jason Hanna and Ashleigh Banfield contributed to this report.