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The Zodiac Killer murdered at least seven people and possibly up to 20 in northern California between December 1968 and October 1969 and it has remained one of the world's greatest unsolved crimes.

The case has spawned dozens of books and a film, starring Jake Gyllenhaal and Mark Ruffalo.

(Image: Kimberly McGath)

The Zodiac nickname was derived from a series of cryptic and taunting letters which were posted to the media from the San Francisco Bay area.

They included ciphers and only one of the four has been decoded.

But now a former detective, Kimberly McGath, has identified the Zodiac as Dennis Rader.

Rader, now 70, is better known as the BTK Killer.

He derived his grisly nickname from the fact that he Bound, Tortured and Killed his ten victims in Wichita, Kansas between 1974 and 1991.

Rader was in the US Air Force from 1966 and 1970 before settling down with a wife in Kansas.

The BTK Killer sent numerous letters to the media but suddenly stopped.

(Image: Getty)

He then vanished until 2004 when he suddenly started sending letters again.

The following year Rader was arrested after he sent police a floppy disk, which was traced back to him.

Faced with a mountain of forensic evidence Rader confessed to being the BTK Killer and is now serving life imprisonment with no parole.

But he has never been questioned about the Zodiac killings.

(Image: Kimberly McGath)

Ms McGath has been studying links between the two killers for two years, especially the letters they both wrote, and she told Daily Star Online : "There are just so many similarities between Zodiac and BTK."

She said the authorities ruled out a link because of a "general alibi" assuming Rader was serving in the US air force in Japan at the time of Zodiac's crimes.

But she said he could easily have travelled back to California to carry out the killings and she points out in her book how close many of the killings were to air force bases.

She lists dozens of similarities in handwriting styles, phrases used and military terminology, as well as phrases like "consternation" and "glorification" which are unique to Lutheran sermons. Rader was president of his local Lutheran church.

(Image: IG)

Both killers referred to "the game" and "hunting" and both used the same ruse to try to calm victims.

A survivor of the Zodiac told how he had said: "I am a wanted man out of Montana and I need your keys and wallet to go to Mexico."

Rader, as BTK, told a victim: "I am a wanted man out of California and I need your keys and wallet."

(Image: Getty)

The Zodiac Killer stabbed or shot his victims out in the open while the BTK Killer targeted people in their homes and often strangled or hanged them.

But Ms McGath explains the change of modus operandi: "When he was in California he was an out-of-state stranger but in Wichita he was well known and could not risk being recognised. That's why he switched to home invasions.

"The only ones who could identify him were the victims and he intended to kill them anyway."

She said she believed he was a "Jekyll and Hyde" character who could be both good and truly evil and she said his mistake in sending the floppy disk – after first asking a BTK detective in a letter if it could be traced – suggested he may have sub-consciously wanted to be caught.

(Image: Getty)

Ms McGath, who lives in Florida, said she had reached out to the police in Kansas and California but they had given her short shrift.

"So many people have put out theories about who the Zodiac is that I think they are just burnt out," she said.

Over the years there have been dozens of theories about the Zodiac Killer's identity including a man who claimed in 2014 that his biological father was responsible.

But Ms McGath said Rader had colon cancer so it was crucial to talk to him before he died.

(Image: Getty)

She said: "If a skilled investigator spoke to him I think they could get him to confess to Zodiac.

"He probably does not want to do further damage to his family but if they used the tactic of religion and persuaded him that he will face eternal damnation if he does not get it off his chest and give closure to the victims."

Ms McGath said she believed Rader had also killed other victims, including prostitutes, while serving in the air force in Turkey, Greece and Okinawa.