North Korean leader Kim Jong Un's bodyguards sparked international attention after they were seen running beside his car at a summit in Singapore last year.

The elite force is chosen from North Korea's most loyal political families, a new book says.

US President Donald Trump made a historic visit to North Korea over the weekend.

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US President Donald Trump made a historic trip to North Korea over the weekend, meeting North Korean leader Kim Jong Un on his home turf. The meeting offered a view of the secretive Hermit Kingdom, including a glimpse of the protection apparatus that ensures Kim's safety.

Kim's life is shrouded in mystery, and his bodyguards remain a source of endless fascination. During Kim's visit to Singapore last year for talks with the US, images of a dozen of Kim's bodyguards running alongside his car made headlines.

"The Great Successor: The Divinely Perfect Destiny of Brilliant Comrade Kim Jong Un," the new book by the Washington Post's Beijing bureau chief, Anna Fifield, says that "Kim got the idea for this human shield from Clint Eastwood."

North Korean bodyguards running along a limousine transporting Kim in Vietnam. REUTERS/ Stringer

"As a boy, he'd seen the movie 'In the Line of Fire,' in which Eastwood plays a US Secret Service agent who had been guarding John F. Kennedy when he was assassinated in 1963. Eastwood's character and other agents run alongside the president's car," Fifield's book says.

Becoming a bodyguard for the North Korean leader is extremely difficult. Lee Yeong Guk, a bodyguard for Kim's father, Kim Jong Il, has said it's "harder than passing through the eye of a needle."

Recruits are chosen from the military and must pass through several tests to judge their health, eyesight, looks, personality, and family background, according to Fifield.

"Those charged with guarding the Brilliant Comrade must have excellent political credentials and come only from the most loyal classes," Fifield's book says. They must be around the same height as Kim, and they are among the only people allowed to carry firearms near the leader.

Kim in Russia in April. Shamil Zhumatov/Reuters

Lee told ABC News in 2018 that guards "have to be good at shooting guns."

"Then Taekwondo, things like throwing knives, swimming, and marching, these are the first," he said. "And the second is serving Kim Jong Il with loyalty, that's also the third and the fourth."

Though the bodyguards, who come from the Main Office of Adjutants, or Central Party Office No. 6, are armed with handguns, they primarily depend on their observational skills and nonlethal methods — like using their hands and bodies — to deal with threats, according to the BBC.

The bodyguards are part of the larger Supreme Guard Command, a force of about 100,000 that navigates several security threats that Kim might face, Time reported, citing North Korea Leadership Watch.

North Korea takes extraordinary measures to protect Kim, including taste-testing food before it's served to him and toting around his personal toilet so no foreign government can analyze his DNA for health information.