Kids in a northern Virginia public school district unknowingly rode a school bus this week that was also carrying “explosives training materials” left behind by the CIA after it conducted exercises with local law enforcement agencies, officials acknowledged Thursday.

The materials were packed in a container and placed in the engine compartment of a bus at Briar Woods High School in Loudoun County on March 24, officials said. It was discovered nearly a week later during a maintenance check of the bus – which by then had carried dozens of kids, including elementary school students, for at least two days.

“During the exercise, explosive training material was inadvertently left by the CIA K-9 unit in one of the buses used in the exercise,” the CIA said in a statement, which also stated the materials did not pose a danger to anyone riding the bus.

After Wednesday’s discovery, local law enforcement officials and eventually the CIA were called to retrieve the materials. Both the CIA and local officials, including the Loudoun County Sheriff’s Office and the Fire Marshal, which had also taken part in the exercises, declined to give a more precise description of the materials after meeting with school officials Thursday.

“The exact nature of the training material used in this exercise is not being released at the request of the CIA so as not to compromise its training techniques,” county officials said.

Officials said the material had fallen out of the container and into the engine compartment, where it went uncollected – and unnoticed - for nearly a week.

Last week was spring break in the district, but the bus was in service on Monday and Tuesday, making eight runs totaling 145 miles, and carrying 26 students attending Rock Ridge High School, Buffalo Trail Elementary School and Pinebrook Elementary School, school officials said.

After the discovery, school officials sent out an email to parents of students in the district, but the alert was short on answers and left many parents wanting a fuller explanation.

“During a routine maintenance procedure Wednesday, a Loudoun County transportation employee found an explosive training material on a school bus used in an exercise by a Federal Government agency last week,” the initial email stated. “The explosive training material was in a benign state and could not be activated through normal operation of the bus.”

One parent of a Buffalo Trail pupil, who asked not to be named, told FoxNews.com she did not understand why the CIA was conducting training on school grounds, and said she was dubious about the explanation of what was left behind.

“What are ‘explosives training materials’ anyway?” she said. “That could mean a lot of things, none of which belong on a school bus.”

CIA officials said the training was routine and part of exercises its personnel do with local law enforcement throughout the metro area.