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EXCLUSIVE: John Douglas has sat down with some of the most infamous serial killers in history - but White supremacist shooter Joseph Paul Franklin was one murderer that truly left him disturbed.

"The victim could have been anyone," the former FBI special agent told Fox News. "He targeted interracial couples, Jewish people, Black people. Just his attitude alone was disturbing. Many of the murderers I’ve interviewed, there’s usually a sexual bent to their crimes. Sometimes they’ll show emotion, even if it’s BS. But he was a different animal. I was hoping for a faint glimmer of remorse. Instead, he was always bragging, boasting, even trying to educate us on how he planned all his crimes."

Douglas, whose work profiling serial killers inspired Netflix’s true-crime drama "Mindhunter," recently co-wrote a new book focusing on Franklin titled "The Killer’s Shadow: The FBI’s Hunt for a White Supremacist Killer." The 75-year-old, along with Emmy Award-winning documentary filmmaker Mark Olshaker, explored what caused Franklin to kill and how he was finally captured.

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Douglas Olshaker (left) and John Douglas teamed up for a new book. (Douglas Olshaker author photo credit Philip Bermingham)

In 2013, Franklin was executed in Missouri for killing Gerald Gordon in a sniper shooting at a suburban St. Louis synagogue in 1977. The 63-year-old was convicted of seven other murders across the country and claimed responsibility for up to 20 overall. The Missouri case was the only one that brought a death sentence.

When studying Franklin, Douglas said there were some red flags in his upbringing that couldn't be ignored.

Franklin, who was born James Clayton Vaughn Jr. in Mobile, Alabama, was the eldest son of an alcoholic drifter who frequently abandoned his family for months or years at a time. He was beaten by both of his parents, with Franklin getting the worst of the punishments among his siblings.

"My mother was full-blooded German who was 5-foot-9 and weighed about 170 pounds," he later claimed, as quoted by St. Louis Post-Dispatch. "She could really hit hard with her hands and she liked to beat us with her hand, slap us, hit us with switches or belts. One time she took a long stick, 2 1/2 feet long and about 2 inches in diameter. I had gone into the fridge and took some of the milk out to pour into a dish for a cat I’d brought home… When she found out, she took that stick and started hitting me as hard as she could."

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Joseph Paul Franklin in booking photos taken in 2005, 2007, and 2012. (Reuters)

According to Douglas’ book, Franklin suffered an eye injury as a young child which the matriarch refused to treat. He was left with severely impaired eyesight.

"It only made him despise his mother more," Douglas explained. "He ends up dropping out of high school. He wants to become a police officer, but he can’t. So he’s feeling inadequate and is filled with rage. He then becomes gravitated towards extreme groups like the American Nazi Party and the Ku Klux Klan. But he decides that’s not enough and wants to take matters into his own hands. This was the birth of a lone wolf."

Franklin, a paranoid schizophrenic, was in his mid-20s in 1977 when he began drifting across America, robbing up to 16 banks to fund his travels. By then he had changed his name to Joseph Paul Franklin after Adolf Hitler’s minister of propaganda Joseph Paul Goebbels and Benjamin Franklin. 

John Douglas served as the inspiration for Netflix's 'Mindhunter.' (Netflix)

In 1977, he bombed a synagogue in Chattanooga, Tenn. No one was hurt, but the killings soon began. That same year, he picked out a synagogue from the Yellow Pages in St. Louis and fired five shots at the parking lot after a bar mitzvah. One struck and killed Gordon, a 42-year-old father of three.

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Franklin got away and his killing spree continued for another three years. He shot and killed, among others, two Black children in Cincinnati, three female hitchhikers, and a White 15-year-old prostitute, with whom he was angry because the girl had sex with Black men. He finally stumbled after killing two young Black men in Salt Lake City in 1980. He was arrested a month later in Kentucky, briefly escaped, and was captured for good after that month in Florida.

"There was no remorse when I interviewed him," said Douglas. "His only disappointment was that he didn’t kill more people. He felt accomplished in life by killing people… He felt it was important to carry the torch, to carry out this act of hatred… Deep down, he hoped to create a race war for others to carry the torch for him. He admired Charles Manson, whom I interviewed because he felt it was remarkable that Manson could get other people to carry out his dirty work to launch a race war."

Larry Flynt, Hustler magazine owner, was taken to Emory University Hospital for further treatment of his gunshot wounds. Flynt and his attorney were ambushed outside a Lawrenceville, Georgia courthouse. (Getty Images)

Years later, in federal prison, Franklin admitted to several other crimes. He was sentenced to death in 1997.

One of Franklin’s most high-profiled crimes was the 1978 shooting of Hustler magazine publisher Larry Flynt which left him paralyzed. The scene was depicted in the 1996 movie "The People vs. Larry Flynt."

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"Larry Flynt put an interracial couple in the magazine," Douglas explained. "That enraged Franklin. He happened to be in the same area as Larry because he was going to trial for obscenity surrounding his magazine. And that was public information. Franklin found a place right across the street, a spot no one was occupying. He sees Larry with his attorney heading to lunch. Then he shoots Larry multiple times."

Franklin also wounded civil rights leader Vernon Jordan in 1980. 

According to John Douglas, Joseph Paul Franklin was fascinated by Charles Manson (pictured here). (Photo by John Malmin/Los Angeles Times via Getty Images)

Just days before he was scheduled to be executed, Franklin expressed remorse for his crimes, St. Louis Post-Dispatch reported. However, Douglas wasn’t buying it. 

"He only showed remorse towards the end and it’s because they were getting ready to fry him," said Douglas. "When I interviewed him he was still fighting the death penalty. He spoke [to us] about his shootings like he was hunting deer. He laughed and joked."

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"He didn’t care who the people were because, in his mind, they were the enemy," Douglas shared. "He wanted people to listen, but he wasn’t that articulate, he certainly wasn’t charismatic. He was a paranoid lone wolf huffing and puffing because he didn’t trust anyone and was filled with anger. And then once Larry Flynt said he didn’t want Franklin executed because he didn’t believe in the death penalty, Franklin then says he respects the guy."

John Douglas' new book 'The Killer's Shadow' is currently available

Douglas hopes his book will shed insight into a killer that has haunted him for years.

"Maybe if he was taken out of his environment as a young child, his life would have taken a different turn," said Douglas. "But he was certainly different. And that’s what makes him so disturbing."

The Associated Press contributed to this report.