Not exactly welcome news for parents.
Though it "could not be activated through normal operation of the bus," the agency said, some residents were not pleased.
"It's very scary," Kalbana Mansabdar told CNN affiliate WJLA
The spy agency used Briar Woods High School in Ashburn, Virginia, for canine explosive detection training while the campus was closed for spring break.
A container holding the explosive material was hidden in the bus's engine compartment.
However, some of the material seems to have fallen out of the container and was accidentally left behind when the exercises ended, according to Loudoun County Public Schools and the Loudoun County Sheriff's Office.
The material was found Wednesday during a routine maintenance check. The Loudoun County fire marshal and Sheriff's Office were immediately called, and the CIA was notified.
The bus transported 26 students from two elementary schools and one high school over the two days the material was left in the bus, schools spokesman Wayde Byard said.
It made eight runs and logged about 145 miles those days. The school contacted parents of students who rode the bus individually.
Byard said the material would need to be placed into a device in order for it to be detonated.
Don't worry -- The CIA "coordinated closely with local authorities and recovered the training material," the agency said in a statement provided to CNN. As a precaution, three local canine units inspected other buses that were parked at Briar Woods and found no additional traces of the explosive material.
It's not clear if they were the same dogs used in the initial exercise, and officials have not disclosed the exact nature of the material.
The Loudoun County school system, Fire Marshal's Office and Sheriff's Office participate in CIA K-9 training. After this week's incident, however, training exercises among groups in this program were suspended.
We hope the dogs who missed it in the first place don't get in trouble.