WASHINGTON — Loudoun County Public Schools says after a training exercise last week, the CIA left behind an explosive substance on one of its school buses.

During the federal training exercise of the sheriff’s office K-9 units at Briar Woods High School, the CIA “inadvertently” left behind what’s being called a container of an explosive material, the agency says.

“After the exercise was over, one piece slipped out of the container and into the engine casing and that’s where we found it this week,” says Loudoun County Schools spokesman Wayde Byard of the substance discovered Wednesday during routine maintenance.

Byard says the unnamed substance was harmless without a specialized blasting cap to activate it.

“So its not something that if you picked it up off the street you could blow up,” he says.

The bus made a total of eight trips on March 28 and 29, carrying children to and from school with the substance on board, the CIA confirms in a release.

The agency says it coordinated closely with local authorities and recovered the training material Wednesday, March 30.

Following an email to parents, the principals of Rock Ridge High School, Pinebrook Elementary School and Buffalo Trail Elementary School contacted the families of the 26 kids who rode on the special needs bus for the two days before it was discovered.

County schools, the CIA, the Loudoun County Sheriff’s office and the fire marshals who participate in the training, jointly agreed to suspend the program for a review of policies that led to the failure to ensure it doesn’t happen again, Byard said.