How reports of Kim Jong Un's health spread and what they tell us about what comes next for North Korea

North Korea's young leader was absent from celebrations marking the country's most important holiday. That much is clear.

But that absence has now fueled speculation that something must be wrong in Pyongyang, with rumors focusing in on the 36-year old's health and his possible demise.

The death of Kim Jong Un could upend a country that has only ever had three leaders -- him, his father and his grandfather -- and possesses a nuclear arsenal that threatens its neighbors, such as South Korea and Japan, and its far-away adversaries, like the U.S.

Some analysts fear that if Kim were to die without a clear heir, it could lead to in-fighting or regime collapse in a worst-case scenario, with the threat of loose nuclear weapons and destabilizing flows of desperate North Korean refugees into China and South Korea.

"It's too early to talk about that because we just don't know, you know, what condition Chairman Kim is in," Robert O'Brien, President Donald Trump's national security adviser, told reporters Tuesday.

Kim was last seen at a politburo meeting on April 11, leading the ruling party's discussion on prevention of the novel coronavirus, according to photos published by state media. But days later, when North Korea test-fired a barrage of missiles, there was no state media report on it and no indication of whether Kim was on hand to oversee it, as he usually is.

The next day, he was again absent from state media for the country's most important holiday, the birthday of his grandfather and North Korea's founder Kim Il Sung. Every year since he took power in 2011, Kim has led a delegation of senior party officials to visit Kim Il Sung's mausoleum -- but neither he nor his sister Kim Yo Jong attended that key ceremony.

Kim has disappeared from public view for a long stretch once before -- in 2014, for what was later reported as ankle surgery -- reappearing after nearly six weeks and walking with a cane. But to miss those events is an indication that "something is off," according to Jung Pak, a former senior CIA official.

"He relies so much on his grandfather's legacy in trying to look like him and act like him that it's unprecedented that he did not show up on April 15," added Pak, now a senior fellow at the Brookings Institution and author of the new book "Becoming Kim Jong Un."

In the days since, South Korean media has swirled with theories on why, including that he has had surgery, that something went wrong and even that he's now brain dead.

While one of the world's youngest heads of state, Kim is in poor health. At 5'7", he reportedly weighs nearly 300 pounds, which, combined with his heavy smoking habit, endangers his health.

The latest rumors came to a head Monday with a report by Daily NK, a South Korean news site founded by North Korean defectors and funded in part by the U.S.'s National Endowment for Democracy, that Kim was receiving treatment after undergoing a cardiovascular procedure, citing North Korean sources.

Western outlets, like Reuters, picked up that Daily NK report, with CNN then reporting that a U.S. official told the network the government was tracking intelligence that Kim's health is in "grave danger" -- although it's unclear whether that intel was something specific officials had learned or the same open-source reporting by South Korean media outlets like Daily NK.

Shortly after, South Korea's president's office pushed back. A spokesperson for the Blue House, as the office is known, said Tuesday, "We have nothing to confirm and there is no special movement that is noticeable within North Korea."

A Blue House official went further, telling ABC News, "What we know now is that Chairman Kim Jong Un is in a suburban area together with close aides and seems to be in normal condition."

At the White House, O'Brien indicated the U.S. doesn't have firm intelligence one way or the other.

"It's just hard to know, but we're keeping a close eye on it," he said.

North Korea is notoriously difficult for foreign intelligence to penetrate.

"Intel from North Korea is so spotty that most (human intelligence) we get is based on rumors from defectors and hushed speculation," said Stephen Ganyard, a former deputy assistant secretary of State and now an ABC News contributor.

But U.S. officials are working now to determine what is going on inside the country and its upper levels of power. Beyond his current state, officials will be focusing in on determining the regime's succession plan.

Since its founding in 1948, the country has been ruled by the communist party, with a dynastic cult of personality at the very top and no clear succession plan beyond the Kim family. But the regime's survival relies on a calm transition between leaders, and whether or not the U.S. knows who, it is likely the regime has a successor in mind, according to analysts.

Kim's own children are too young; the three of them have never been mentioned in state media and are thought to be 10 years old and younger.

He has also spent his near decade in power consolidating control and eliminating potential rivals -- having an uncle executed by anti-aircraft gunfire and a half brother assassinated using a chemical weapon. He has another brother, Kim Jong Chol, but he has no official title and hasn't been seen publicly in the country in years.

There is one family member, however, that Kim has drawn in closer to his political orbit -- his sister Kim Yo Jong. During his last public appearance at that politburo meeting, she was reinstated in a senior party role, as an alternate member of the political bureau. She has traveled to South Korea and Singapore, where she accompanied her brother for his first meeting with Trump, and in recent weeks, state media has issued statements on foreign policy in her name -- a new sign of her speaking for the regime.

"Kim Yo Jong has gained authority and exposure in recent months and years, perhaps sufficient to be the next leader to maintain the Kim bloodline," said Bruce Klingner, a former senior CIA official and now a senior fellow at the Heritage Foundation, who added that Kim Yo Jong has another key qualification: "She is likely the only person he trusts."

It's possible a patriarchal society like North Korea's would disprove of a female leader, but according to Pak, it's her family ties that matter most.

"The Kim family has done a great job of inculcating this idea of the blood line being holy. ... Blood would trump gender," Pak said.

Whether Kim retains power, is in a fragile state or dies, North Korea's nuclear weapons program will continue to threaten not just Americans in the region, but likely the entire continental U.S. at this point.

While Trump has touted his personal meetings with Kim as the solution to that threat, North Korea's nuclear arsenal has only grown since he took office, and the diplomatic process has been dead since his working-level negotiators met Kim's last October in Stockholm, Sweden, and both sides refused to move first.

Who is in control in Pyongyang is unlikely to change the regime's assessment that those nuclear weapons are critical to their survival.