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Rusty Young has crossed paths with some of the world’s most dangerous people.

The Australian journalist spent four months inside a Bolivian prison where he met murderers, rapists and made friends for life with a drug smuggler who later named his first born “Rusty.”

But there’s one interview that made the best-selling author of "Marching Powder" nervous.

On Friday, he told news.com.au what it was like to meet a man who admitted to murdering 300 people with his own hands — and who is believed to have been responsible for the deaths of almost 3000 more.

In the making of Young’s new documentary, "Wildlands" — which explores the Bolivian drug trade and will be released on March 6 — the author-turned-filmmaker found himself face-to-face with John Velasquez.

The man known throughout South America as “Popeye” was Pablo Escobar’s right-hand man and, according to Young, the “deadliest hit man in cartel history.”

Having just been released from prison, Young tracked Velasquez down via email and the pair shared some “friendly and courteous” conversations.

But hours before filming began, the gravity of the situation began to set in.

“The guy’s a killer,” Young told news.com.au. “The guy’s killed so many people and I realized I didn’t know whether he’s going to be carrying a gun."

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