LAWRENCEVILLE, Ga. — The Gwinnett County Police Hazardous Device/Explosive Ordnance Disposal Unit is investigating a suspicious device that was placed on a school bus as it was stopped in traffic on Wednesday.

The Gwinnett County Police Department said that it received a call from a passerby reporting that she observed a female exit her vehicle and place an object underneath a Gwinnett County Public Schools bus as it was stopped for traffic at an intersection. The female returned to her vehicle and drove off.

As the school bus drove off, the suspicious object fell from the bottom of the bus onto the roadway. The bus in question was located when it arrived at Minor Elementary School in Lilburn. As a precaution, the school was locked down for a short period of time while technicians from the Hazardous Device Unit searched the bus for any suspicious devices or objects.

Meanwhile, police set up a perimeter around the object that fell from the bus, closing the intersection from traffic and pedestrians for further investigation into the object and its contents.  

A thorough search was conducted on the bus, and no suspicious devices or objects were found. The object that fell from the bottom of the bus turned out to be a thermos that contained water. Authorities were not yet sure what the motive was or why the female placed the thermos underneath the school bus.

Investigators with the Gwinnett County Police Department, FBI and Gwinnett County Fire/Arson are conducting "a thorough criminal investigation into the highly suspicious circumstances of this event," the police department said in a statement.