Following a training exercise last week, the CIA "inadvertently" left explosive material under the hood of a school bus in northern Virginia.

With the material still sitting in the engine compartment, the bus was then used to ferry elementary and high school students to and from school Monday and Tuesday, according to the CIA and law enforcement officials in Loudoun County, Va.

On Thursday, the Loudoun County Sheriff’s Office and the CIA said in statements that the explosive material, which the agency says posed no danger to passengers on the bus, had been left behind following a training exercise at Briar Woods High School.

The CIA said that the exercise, which was conducted during spring break, was a routine training scenario for explosives-detecting dogs.

The explosive was a "putty-type" material designed for use on the battlefield, Loudoun County Schools spokesman Wayde Byard told The Washington Post.

Putty, or plastic explosives, which require a special detonator and are often used in demolition, are considered stable, the Post reports.

When law enforcement agents used the bus to conduct a training exercise last week, CIA trainers placed explosive material into the engine compartment of a bus, as well placing the material into some parts of the school, Mr. Byard said.

The dog successfully found the material in the engine compartment, but some of the material fell deeper into the compartment and became wedged beneath the engine’s hoses.

But while bus drivers check under the hoods of their buses before they drive them each day, the package was wedged too deep inside the engine compartment to be seen, he told the Post.

Before the material was discovered during a routine maintenance check by a technician, the bus made eight runs totaling 145 miles while carrying 26 students over the two-day period.

"We’re all very upset by what happened, but we're going to review everything that did happen," Byard said, noting that the school officials and local law enforcement had met on Thursday and determined that all training exercises at school facilities would be suspended until stronger protocols could be put in place.

"Obviously we're concerned. The CIA really expressed its deep concern and regret today, and it was sincere," he said.

Military training exercises have occasionally gone wrong in the past – one that used live ammunition at a base in Nevada killed seven in 2013. To combat those dangers, the US Marines have also been conducting tests with a robotic bomb-sniffing dog called Spot, developed by robotics company Boston Dynamics.

But live dogs have increasingly become a fixture at a range of train stations and airports, in addition to post offices, sport stadiums and banks, renowned both for their keen sense of smell and handlers' ability to train them for a variety of situations.

"Strictly speaking, the dog doesn’t smell the bomb," wrote Smithsonian Magazine’s Joshua Levine in 2013.

"It deconstructs an odor into its components, picking out just the culprit chemicals it has been trained to detect. [Zane] Roberts [who trains bomb-sniffing dogs for a private company in Connecticut] likes to use the spaghetti sauce analogy. 'When you walk into a kitchen where someone is cooking spaghetti sauce, your nose says aha, spaghetti sauce. A dog's nose doesn't say that. Instinctively, it says tomatoes, garlic, rosemary, onion, oregano.' It’s the handler who says tomato sauce, or, as it happens, bomb," the magazine reports.

Get the Monitor Stories you care about delivered to your inbox. By signing up, you agree to our Privacy Policy

On Thursday, the CIA said it had recovered all the training explosives used in Loudoun County.

"To prevent such incidents from happening again, CIA has taken immediate steps to strengthen inventory and control procedures in its K-9 program," the agency said in its statement, referring to the program for bomb-sniffing dogs.