GRAY, Ky. — A school bus monitor was critically injured when she fell from her bus as it was moving on Wednesday, police said.

The Kentucky State Police reported that around 4 p.m. on Wednesday, a clipboard that had been stored under the dash fell from the storage compartment and landed in the stairwell of the Knox County Public Schools bus. The bus monitor, Sueanne Sizemore, 63, left her seat and entered the stairwell to retrieve the clipboard.

The bus driver, William Reeder, told investigators that he glanced over to make sure that Sizemore did not lose her footing while leaning over. When he moved his eyes back to the roadway, the bus had left the lane of travel. Reeder said that the bus was about to strike some mailboxes, and he swerved to avoid impact.

Police said that the sudden maneuver caused Sizemore to lose her balance and fall through the closed bus doors. She struck the pavement and rolled into a ditch, leaving her unconscious.

Sizemore was airlifted from the scene to a hospital in Lexington, Kentucky. She was listed in critical condition.

Investigators said that the bus doors require 43 pounds of pressure to be forced open without the use of the opening lever.

“Once Sizemore fell against the doors, they easily opened, allowing her to fall from the bus,” Kentucky State Police officials said in a news release. “Once the force is removed, the doors automatically return to their closed position.”

The incident is being investigated by Trooper Sammy Faris.