RODRIGUEZ LIVE FIRE RANGE, South Korea — The U.S. soldiers donned gas masks as they cleared building after building after receiving intelligence that North Korean scientists were cooking up chemical weapons in the vicinity.

They found the makeshift lab with beakers and tubes filled with a yellow substance in a room off the icy rooftop of a two-story brick building. It was time to call the 501st CBRNE Company (Technical Escort), a specialized unit trained to deal with weapons of mass destruction.

The scenario was part of a training exercise, but the dangers facing the soldiers stationed near the front lines of the divided peninsula are all too real.

North Korea has demonstrated alarming progress in its nuclear weapons program this year, with two underground atomic explosions and two dozen ballistic missile tests.

But the Stalinist state is believed to have vast stockpiles of other nasty stuff, too.

“North Korea has the full spectrum of all types of chemical and biological weapons … and they’ve weaponized all of it,” Lt. Col. Roberto Salas, commander of the 23rd Chemical Battalion, told Stars and Stripes.

He said troops train for a variety of possible hazards, including choking, blister and nerve agents; and biological threats such as anthrax, smallpox, cholera, botulism and fevers.

“But we believe that we’re ready to fight tonight and respond to anything that they would use — the full spectrum of anything that they decide to use,” he said.

‘Mad scientists’

The Camp Stanley-based battalion — originally formed in 1944 to generate smoke for cover during World War II — is the only forward-deployed unit of its kind, underscoring the high threat level.

Its mission is to counter chemical, biological, radioactive, nuclear and high-yield explosive threats — thus the acronym CBRNE.

The target on Dec. 7 was the lab in a brick shell of a building in a mock village that is used for training at the Rodriguez Live Fire Range, a massive complex about 15 miles from the world’s most heavily fortified border.

Apaches buzzed overhead and fired into a nearby mountainside as part of a separate exercise.

After searching for days, soldiers with the Fort Riley, Kan.-based 3rd Battalion, 66th Armored Regiment, 1st Armored Brigade Combat Team, 1st Infantry Division, found the lab, detained suspects and made sure the area was clear of explosives.

They also found warheads that were apparently to be used to weaponize the chemical agent.

Maj. Michael Tiongco, 34, of Hawaii, said the lab was built to be as realistic as possible.

“They didn’t know what it was going to look like. They didn’t know what they were going to see,” he said of his team, adding that a chemist, a facilitator and a security guard were taken into custody.

“They captured, so to speak, the mad scientists,” he said.

The 501st CBRNE Company, meanwhile, set up a decontamination area outside with a red rope marking the danger zone and a blue rope for the safe side.

Team leader Sgt. Jonathan Simmons, 34, of Chicago, climbed the stairs with a translator to examine the lab’s contents and papers that had been left behind.

“It was a mustard agent,” he said, adding that the papers mostly contained North Korean propaganda against the South.

“We won’t always know what the final product is,” he said. “It can be kind of touch-and-go sometimes, but I’m pretty confident about this one.”

The soldiers radioed the details to headquarters and received orders about which samples should be collected.

Capt. Kim Sae, the company commander, said a top priority was to secure the samples and make sure they’re not contaminated in case they need to be used as evidence in the future.

“That’s why we keep a very strict chain of custody. It’ll go through a series of handovers, and during those handovers, we keep a very close log,” he said. “The end goal is to get a field comfirmatory about what is here, then senior commanders evaluate things.”

The final step was for everybody to line up behind the red rope and go through a rigorous decontamination process that included having their special protective suits cut off near a red fire truck.

While last week’s exercise involved only the combined U.S. units, Salas said they also frequently train with their South Korean counterparts.

“In addition to our capabilities, the Republic of Korea capabilities are solid,” he said.

The battalion also has the use of some of the most sophisticated equipment for detection, analysis and protection, he said. That includes 15 specially fitted Strykers with sensors to detect a CBRNE threat, and a remote-controlled, .50-caliber machine gun to enable it to fight at the same time.

Salas, who took over the command six months ago, expressed concern that the use of chemical weapons in Syria and elsewhere in the world could embolden North Korea.

“Every day, the proliferation of this information and technology and the availability of things online to be able to do something is just made more available,” he said. “So we think the recent use (of chemical weapons) throughout the world could probably embolden (the North) — say, ‘Hey, could I use this to reach a strategic or tactical objective?’ — and I think they’re well aware of that.”

Extent unknown

The extent of North Korea’s biological and chemical weapon stockpiles — and the development of missiles to deliver them — is a matter of debate because of limited intelligence about the reclusive country.

Pyongyang denies having them, in contrast to its proud acknowledgment of its nuclear weapons program.

The South Koreans aren’t taking any chances, installing glass cabinets with emergency gas masks in subway stations to be used in case of toxic gas attack, or chemical, biological or radioactive contamination.

In one of the most detailed recent assessments, weapons of mass destruction expert Robert J. Peters said a reasonable guess of the country’s annual production capability of chemical weapons would be in the low tens of thousands of metric tons of material.

“It is safe to assume that the North has been producing first-generation blister, choking and nerve agents, and conceivable that they have a limited number of more advanced binary agents,” such as VX or Sarin, Peters said in a paper published last year on 38 North, a U.S.-based website that monitors the country.

Peters said the North Koreans probably have artillery shells armed with chemical weapons and possibly bulk agent positioned north of the heavily fortified Demilitarized Zone that divides the peninsula.

“It is also possible that such shells and bulk agent are located elsewhere in the country, which would further complicate any foreign military movements deep into North Korean territory,” he added.

The makeup of the country’s biological weapons arsenal is less well-known, although strong evidence that it could produce anthrax emerged when leader Kim Jong Un was shown touring a purported pesticide factory in June 2015.

Some North Korean defectors also have claimed the programs date to the 1960s and say the North has used human testing.

Members of the CBRNE unit receive advanced specialized training before being deployed to South Korea and are regularly updated with continuing educational opportunities.

It all boils down to the need for constant training to fight an enemy that has so far largely remained on the other side of the mountains. “Every time we get out here and we’re able to integrate as part of a force in a learning environment … is priceless,” Salas said. “There’s always lessons learned.”

gamel.kim@stripes.com Twitter: @kimgamel