President Donald Trump is looking to score a historic coup by securing a nuclear deal with North Korea - but making sure Pyongyang sticks to the agreement could be even harder.
Trump's advisors, from intelligence officials and foreign allies, to diplomats and military personnel, are concerned that no matter what kind of "deal" the President makes with North Korean dictator Kim Jong Un, it will be incredibly difficult to verify whether he keeps his promises.
The Trump administration will be dealing with one of the most difficult intelligence targets on the planet - a secretive, mountainous place where it's hard to conduct aerial reconnaissance and next to impossible to recruit human spies.
There will be enormous technical challenges: North Korea has a complex nuclear program and a history of lying about it. And getting North Korea to "yes" will take highly skilled diplomacy. Ensuring that the program is truly being dismantled will require convincing Pyongyang to let inspectors have total access to the historically isolated nation.
Largest stumbling block
In more than a dozen interviews, former and current intelligence and national security officials told CNN that reaching agreement on inspections and the continued monitoring of North Korea's nuclear program could be the largest stumbling block in coming negotiations.
"Getting the deal is only half the battle," said a former intelligence official who specializes in Asia. "The other, more difficult half is compliance." Speaking of the intelligence community, this source said, "They are worried."
"It will, indeed, be a heck of a challenge," said Olli Heinonen, the former deputy director general of the International Atomic Energy Agency, the officials trained and tasked with inspecting nuclear sites all around the world, in an email to CNN.
Intelligence agencies won't be the ones to make a conclusion about whether Kim is in compliance, but their work will help diplomats, officials and policymakers reach those decisions, or decide whether further inspections are needed.
Reliable verification and monitoring will also be necessary to provide the President with intelligence about North Korea's nuclear program that is as close to real time as possible. What he does with that information is up to him.
So far, administration officials are sounding a cautiously positive note, even as they acknowledge the verification challenge ahead.
US Secretary of State Mike Pompeo has said repeatedly that the US is "committed to the permanent, verifiable, irreversible dismantling of North Korea's weapons of mass destruction." He's raised the issue with North Korean delegations, but the nitty gritty planning around methods of inspection will ultimately be left to professional sat the State Department and intelligence community.
"We have conveyed to the North Koreans our expectations that they will be fully transparent," an NSC spokesperson told CNN, adding that ensuring compliance will be the work of years.
"Verification is a complex process that requires potentially years of transparency and access to a wide variety of locations and facilities to verify that North Korea has upheld its commitments and not retained nuclear material, weapons, or capabilities in violation of an agreement," this spokesperson said.
"The more forthcoming that North Korea is, the more quickly we will have confidence that North Korea has actually committed to and followed through with denuclearization," the spokesman said.
But there's fear, according to multiple former intelligence officials who maintain contact with ex-colleagues, that even with a robust verification and monitoring regime, North Korea's infrastructure is vastly more complex than the fledgling projects in Iran that the US had to deal when negotiating the Obama-era Iran nuclear deal.
The complexity, along with the Kim regime's history of obfuscating its true plans, will make for a massive challenge, even if everything goes right at the June 12 summit in Singapore and in the months after that.
"The entire deal hinges on verification," said one national security official.
The former intelligence official with experience in Asia said the stakes will be high for their former colleagues. "It falls on the [intelligence community] to get this right," the official said.
"You will find the intel guys pounding the table for intrusive verification," Gen. Michael Hayden, a former director of both the CIA and the National Security Agency, said in a phone interview.
Referring to CIA Director Gina Haspel and Director of National Intelligence Dan Coats, Hayden said, "you would find them the strongest advocates" for a robust monitoring system.
Even if the US secures a deal with built-in verification - and the two sides agree on the definition of "denuclearization" - the process won't be easy.
And there are concerns in some quarters that politics could get in the way.
Some in the intelligence community worry that the Trump administration, eager to cut a good deal and establish the President as a historic peace broker between the Koreas, will put pressure on officials to "find the North Koreans in compliance, so that it won't look like we cut a bad deal," the former official said.
Those concerns were echoed by Senate Majority Leader Mitch McConnell at a congressional summit in Kentucky on June 1.
"I think for these situations to work, you have to not want the deal too much," McConnell said. "If you fall in love with the deal and it's too important for you to get it ... you could get snookered," he said. "I think the President's fully aware of that," he added.
The intelligence community will have its hands full enough confirming the verification process, which will be lengthy.
North Korea will need to produce a full list of its nuclear materials and facilities past and present, in what's called the "declaration" phase.
Next, Pyongyang will need to enact a complete and total freeze of all operations so that inspectors can examine everything, Heinonen explained in a phone interview.
Destruction of one of its testing sites in May, witnessed only by a handful of journalists, already goes against the ideal timeline, because no experts were able to inspect the site first. "From here we need to go in an orderly manner, to be able to verify that this program really has been dismantled in an irreversible way," Heinonen told CNN.
Staffing is another challenge. Work on the ground would likely be done by inspectors from the International Atomic Energy Agency.
But there aren't that many officials trained to do the kind of nuanced, scientific and mathematic work it takes to analyze and monitor nuclear production. The IAEA is strapped for personnel, as it already has hundreds of inspectors working elsewhere around the globe.
It will be a top priority to ensure total access for those inspectors who are sent into North Korea, including safe and efficient transportation around the mountainous terrain.
"Basically, the goal would be to get unfettered IAEA access to the whole country," said one congressional aide familiar with discussions between intelligence officials and diplomats planning for the summit.
And it will be crucial for those inspectors to be able to enter sites that the North Koreans haven't declared.
Access is everything
"A verification regime that will permit challenged inspections of non-declared suspect nuclear sites and facilities would be necessary for any robust verification agreement," said Joe DeTrani, a former senior intelligence official specializing in Korea and a special envoy for the Six Party Talks with North Korea between 2003 and 2006.
That includes North Korean agreement to allow independent experts from outside the country to analyze soil and water samples, he said.
Otherwise, history could repeat itself.
DeTrani recalls that a 2005 agreement in which North Korea promised to give up its nuclear program in exchange for economic, security and energy benefits fell apart because of Pyongyang's unwillingness to sign a verification protocol.
The kind of access necessary to ensure verification won't be possible without cooperation from Pyongyang, said one foreign official involved in the summit.
"There's some accumulated knowledge and history there, but it will be tough ... [and] North Korea probably wants some sovereignty" over how much access is allowed into the country, the foreign official continued. "For any verification, without cooperation from North Korea, almost nothing can be done."
But the Congressional aide said that the US is already considering how to deal with possible North Korean attempts to restrict access to the country or curtail inspectors' ability to do their work.
"There's a lot of thought being given to what other ways we can verify compliance," without total voluntary cooperation, the aide said.
The US does have methods of monitoring the country, from satellites and sensors, to limited amounts of signals intelligence generated when the country's elites go online and from regime hackers, according to public research by Priscilla Moriuchi, one of the NSA's former top officials on East Asia and the Pacific, now with the cybersecurity firm Recorded Future.
But North Korea remains one of the most difficult intelligence targets on the planet.
The US routinely flies U2 planes along the demilitarized zone that divides the two Koreas, but for now, it's difficult to fly those planes over North Korea without them being shot down.
Down the line, according to one national security official, the US would likely demand flyovers using CIA-owned planes equipped with specialized sensors to detect particles in the atmosphere, such as enriched uranium.
Those planes would need to fly lower to the ground in order to pick up those particles, particularly as much of North Korea's nuclear work takes place underground.
While US satellites and those belonging to allies such as Japan can now detect obvious military activity from overhead, North Korea is well aware of their trajectory in the skies above, according to multiple professionals who work in geospatial intelligence who spoke with CNN.
And their response to the surveillance points to how difficult it could be to accurately track what's going on inside the country.
Several officials told CNN about North Koreans playing volleyball and writing "Merry Christmas" on the rooftops of military facilities, knowing that Americans and others were watching from above, hoping to catch wind of sensitive projects.
"The imagery card is always available, but they're aware of that and have gone underground," said Gen. Hayden.