Patillo exited the bus with the woman in the area where the passersby spotted him exposing himself a few minutes later, police officials said.
The Auburn Plainsman, the university's student newspaper, reported that the alleged assault took place on a Tiger Ten bus that runs from the downtown area to multiple apartment complexes and student housing areas off-campus. The late-night buses are specifically designed to give students a safe ride home.
"Our top concern is the well-being of the victim, and we cannot stress in strong enough terms our shock and distress over this despicable act," officials with Auburn's Department of Campus Safety and Security said in a statement. "We immediately provided support and all available resources to the victim and continue to do so."
The Plainsman reported that the university operates campus security shuttles to take students to on-campus locations late at night. Tiger Transit and Tiger Ten buses are operated by outside contractor First Transit.
First Transit is required in its contract with Auburn University to perform background checks on all of its drivers, the campus newspaper said. Company officials told the Plainsman it is performing its own internal investigation of the alleged assault.
"At First Transit, we are greatly troubled by the events of Friday night," officials said in a statement. "The safe and reliable transportation of our passengers is our highest priority. It is a responsibility we take very seriously."
Both Patillo and Johnson were immediately removed from service and First Transit has begun termination proceedings, the statement read. Company officials said they are working with campus and city police in the investigation.
Auburn University is re-evaluating its contract with First Transit, the Plainsman reported.
Patillo was being held in the Lee County Jail in lieu of $127,000 bail, the News reported. Johnson was being held in lieu of $125,000 bail.