The bus then remained in service for two days, transporting students to a high school and two elementary schools, before the explosive material was found during routine maintenance, the sheriff's office says in a statement.
According to a statement from the school district, the bus carrying the material "made eight runs totaling 145 miles carrying 26 students attending Rock Ridge High School, Buffalo Trail Elementary School and Pinebrook Elementary School."
The CIA put the material in the bus as part of a regular joint exercise to train explosives-sniffing dogs at Briar Woods High School on March 21-24, during spring break, the sheriff's office says. The Loudoun County Sheriff's Office and Fire Marshal's Office took part in the drill.
This is how the sheriff's office explains how the material was left under the hood of the bus:
"As part the training exercise last week, canine explosive detection training was conducted in areas inside and outside of the high school. During the outside portion of the training, the training material was hidden inside an engine compartment of the bus. At some point, a portion of the material appears to have been dislodged from the container and fell into the engine compartment of the bus and was not recovered following the training."
It adds that the material is "incredibly stable" and "benign," without disclosing details. Loudoun schools spokesman Wayde Byard tells The Washington Post that it was "a 'putty-type' material designed for use on the battlefield and which requires a special detonator; such putty, or plastic, explosives — including the well-known C-4 — are used in demolition."
The sheriff's office, as well as the CIA, said students were not in danger and that "parents of students who might have ridden the bus while the training material was on board were contacted on March 30."
But the lapse raises questions about how the oversight could have happened.
Now, Loudoun County officials say the training program has been suspended pending a review. The CIA says it is also conducting an independent review of the K-9 program and beefing up its inventory and control procedures.
Local officials say other buses "that were used or were near the training exercise" have been searched as well.