Utter devastation lies before investigators days after a building at a Tennessee explosives plant was razed by a blast so powerful that it left behind no living witnesses, created a vast and volatile investigative scene and cast potential evidence for miles around.

The cause of Friday’s explosion is still under investigation, but authorities caution answers may not come for weeks or months due to unique challenges that will require a delicate, methodical approach and “minute-to-minute” safety evaluations.

Undetonated explosives pose a serious threat to first responders and investigators. The manufacturing facility, Accurate Energetic Systems, lies roughly 60 miles west of Nashville and crafts highly explosive products for the military and industrial companies.

Before any true investigative work or victim recovery can begin, the blast area must be combed “foot by foot” to ensure explosives have been safely neutralized, Gov. Bill Lee said over the weekend.

“Once we have that entire area cleared for all hazards, all remains, everything else, then we’ll start the post-blast investigation to see the cause and origin of what happened,” said Brice McCracken, special agent in charge at the Bureau of Alcohol, Tobacco, Firearms and Explosives’ National Center for Explosives Training and Research.

But finding the root cause of the explosion will also be a painstaking task.

The initial blast triggered a cascade of smaller explosions, creating about half a square mile of damaged area and potentially obscuring the true origin of the blast, authorities have said. Debris that may be key evidence could be as small as a fingernail, experts told CNN, and might have been thrown miles away.

The ATF, which is leading the investigation, has sent in one of its elite National Response Teams.

ATF will ensure “that if criminal activity is involved, those responsible will be held accountable, and if it was accidental, that lessons have been learned to prevent a tragedy like this from ever happening again,” Tyra Cunningham, assistant special agent in charge at the agency’s Nashville division, said.

Here are the key challenges investigators will face – and how they plan to overcome them.

Vast scene is littered with possible landmines

In any explosion investigation, rendering the scene safe is paramount, but the nature of Accurate Energetic’s product increases the risk of harm to exponentially.

Though it is unclear what, precisely, was manufactured in the destroyed building, Accurate Energetic is a key supplier to the military and manufactures bulk explosives, land mines and small breaching charges, including C4. Last month, the US Department of Defense awarded the company a contract for nearly $120 million for “the procurement of TNT.”

Bomb technicians from the ATF, FBI and Tennessee Bureau of Investigation undertook the meticulous and physically grueling task over the weekend.

“We’re working at a snail’s pace,” Humphreys County Sheriff Chris Davis said. He noted the task is “very physically demanding” and technicians are donning heavy protective gear under the mid-day Tennessee heat.

Leading the effort is the ATF National Response Team, a specialized rapid-response force that responds to the nation’s highest-profile bombings, explosions and arson, including the 9/11 World Trade Center attack and the 1995 Oklahoma City bombing.

The National Response Team on site in Tennessee includes certified explosive specialists, bomb technicians, electrical engineers, forensic chemists and intelligence researchers, Cunningham said

“Together they bring decades of experience in determining the origin and cause of explosions and fires,” Cunningham said.

The explosives disposal teams began on the outer fringe of the blast site and are working step-by-step toward the area where they believe the blast could have originated, McCracken, the ATF special agent, said Saturday.

A drone from the sheriff’s office was also used to identify potential “hot spots” from the air,” he added.

“But it’s going to take time to get to the actual facility where they were working with the explosive materials,” McCracken said.

One advantage, Chittum said, is there will be a detailed record of what hazards were being stored inside the building because Accurate Energetic is a government contractor, said Thomas Chittum, who spent more than two decades with the ATF and retired as its No. 2 official.

All 16 people inside the facility died in the blast, leaving behind a devastated network of coworkers and loved ones as well as depriving investigators of crucial accounts of the final moments leading up to the explosion.

“Everyone who is there who could have described what was happening, what went wrong immediately before the blast, is dead,” CNN analyst John Miller said.

Instead, authorities will have to interview other workers who were not there and who may offer insight into the conditions, Chittum said.

“What was the state of the workplace? Was it clean? Was it orderly? Was the company following the law?” Chittum said.

Sometimes, surveillance footage can provide important information about the timing and location of the first blast, Chittum said. However, that footage may not exist in this case due to the scale of the damage, he said.

Identifying victims’ remains has also been a slow process because their bodies cannot be recovered until the surrounding area is declared safe from explosives. While investigators have compiled a list of employees presumed to be dead, the victims’ loved ones must endure the agonizing wait to be united with their remains.

Authorities have tried to expedite the process by getting DNA samples from family members whose loved ones are in the rubble, TBI Director David Rausch said. Once their remains become available, authorities have “rapid DNA” technology that can quickly match their DNA to their family samples.

Once investigators have been cleared to begin analyzing the scene, they will be faced with a chaotic expanse of mangled equipment, shards of manufacturing material and hunks of evidence that have been flung far from its origin. Typically, the first step is to identify the seat of the blast.

“The problem in this scene is that it is complete devastation,” Chittum said. Oftentimes, an explosion will only partially damage a building, making it clear where the blast originated. In this case, the building has been essentially razed.

The already large and complicated scene has also expanded as residents have continued to find unusual debris cast as far as two miles from the facility, Hickman County Sheriff Jason Craft said. He encouraged residents who find suspicious objects to avoid touching them and call 911.

The ATF and assisting agencies will use facility blueprints, pictures and a field of debris to reconstruct the scene, agency Special Agent in Charge Matthew Belew said Monday.

“It’s almost like putting a puzzle back together,” Belew said.

Agents will use a grid system to chart “hundreds if not thousands” of pieces of tagged evidence, he said. They will then try to determine where each piece originated from and the distance and direction that it was thrown by the explosion.

“Literally you’re taking small pieces trying to construct them back into big pieces,” Belew explained.

CNN previously rode along with ATF National Response Team specialists deployed to investigate a suspected arson in Minneapolis, and they spoke about the extreme difficulty of gathering evidence after an explosion or fire.

“There are a lot of scenarios when we go into areas where we have to remove debris and rubble, we might be looking for something maybe smaller than your fingernail,” ATF Supervisory Special Agent Dixon Robin said in 2020.

While gathering evidence to help determine the cause of an explosion or fire, ATF specialists are trained not to rush in drawing conclusions, Robin said.

“If we’re going to give the answer — and our mission is to give an answer — we have to make sure we’ve considered every possibility,” he said.

During the deadly Los Angeles fires earlier this year, ATF NRT team leader Chris Forkner told CNN investigators do feel pressure when working to get the public answers but, he said, “we have to step back, remain objective, remain scientific about our process.”

Forkner added, “We can’t let emotion and community pressure and political pressure drive our investigation. We have one shot to do this right.”

CNN’s Josh Campbell and John Miller contributed to this report.