With his short blonde hair and distinctive thick rimmed glasses, experts believe this could be the face of the notorious Zodiac killer.

The serial killer shot and stabbed five victims to death in a chilling spree that gripped northern California in the late 1960s. Hungry for attention, the killer later sent the press a string of coded notes hinting at his identity but was never brought to justice.

Now nearly 50 years later, two experts believe they might have finally made a breakthrough in the case and think the man responsible could be a suspect who was first questioned by police over the 1966 murder of a teenage girl who has long been considered a possible Zodiac victim.

In a thrilling new documentary, former LAPD homicide detective Sal LaBarbera and cold-case expert Ken Mains look at the evidence from the murder of Cheri Jo Bates, an 18-year-old who was beaten and stabbed to death in California.

They discovered that the original suspect in the case, Ross Sullivan, fits a description of the Zodiac that was given by an eye-witness during the police investigation into another killing. A photo of Sullivan, who would now be 76, shows him wearing a pair of thick rimmed glasses that are strikingly similar to ones seen in a police sketch of Zodiac.

Speaking exclusively to FEMAIL, Mains explained that the time that has passed since the killings means it could be easier to catch the killer, adding: 'Time is an ally'.

Face of a killer? Experts believe Ross Sullivan, pictured, who was a suspect in the 1966 murder of teenager Cheri Jo Bates, could be the notorious Zodiac killer who terrorised northern California in the late 1960s. He was questioned by police over Miss Bates's murder

Police sketch: Experts argue Sullivan bears a striking similarity to the police sketch released of the Zodiac killer, pictured. Sullivan would now be 76 and his whereabouts are unknown

The Zodiac killer has been linked to five murders in a chilling 10-month spree between December 1968 and October 1969 but it is suspected he could have been responsible for dozens more stretching into the early 1970s.

One such suspected victim is Cheri Jo Bates, who was found with stab wounds similar to those of the Zodiac's confirmed victims.

At the time Sullivan was mentioned in the local press as a suspect and was questioned by police.

Why Sullivan is under suspicion

In new documentary series Zodiac Killer: Case Closed?, the two homicide experts discovered that there is enough evidence to prove Bates was murdered by the Zodiac and believe it could point to Sullivan as the main suspect. His current whereabouts are unknown.

In 1968, the year of the first Zodiac killings, Sullivan was arrested for 'bizarre behaviour' in Santa Barbara, not far from where the murders took place.

He also used to work at the university library close to where Miss Bates's body was found.

A sixth victim? Cheri Jo Bates stabbed to death and nearly decapitated on October 30, 1966 at Riverside City college. She was linked to the Zodiac due to the brutal nature of her murder

Experts spoke to former colleagues from the time who described Sullivan as a 'quiet' man who always wore the same army style clothing, complete with military boots.

The Zodiac Killer's reign of terror The serial killer operated in Northern California in the late '60s and early '70s - his identity is still not known. Evading capture, the killer taunted police by sending clues to local Bay Area press in the form of cryptograms, hence the name the Zodiac Killer. Though police linked him to five murders, he boasted of at least 37 victims in his letters to the press. Advertisement

This was striking as the footprint of army boots were found at the crime scenes of the Zodiac's confirmed victims.

The former library worker also looks very similar to the police sketch of the Zodiac that was released to the media when the authorities appealed for information.

Both men wear similar glasses and have similar haircuts.

There are also elements of Sullivan's background that suggest a link.

The Zodiac was known for taunting the police with coded messages that continue to baffle experts more than 50 years later.

Strikingly, Sullivan was an English student who did a research paper on cryptology before the killings started.

Mystery: The case of the Zodiac killer remains unsolved nearly 50 years after the killings

Mains and LaBarbera also examined an advert in a copy of the San Francisco Chronicle from the time for a Zodiac stationary store, which featured striking similarities to the images the killer sent to the police.

The shop also sold tickets to the a local production of the Mikado opera, which was quoted in the Zodiac's letters. The store was located not far from where Sullivan was living at the time.

I think as time goes by time helps you because loyalties change alliances change friendships change. I think time, especially in a cold case investigation, is an ally and it helps you solve cases Cold case expert Ken Mains

Commenting on the evidence, Mains explained that he believes that the time is right to finally uncover the Zodiac killer's true identity and that the more the nearly 50 years since the murders could help detectives find the truth.

He said: 'I think as time goes by time helps you because loyalties change alliances change friendships change. I think time, especially in a cold case investigation, is an ally and it helps you solve cases.'

The documentary also showcases the work of a supercomputer that has been used to try and crack the codes written by the Zodiac.

A University of Southern California professor created an artificial intelligence software that was designed to help crack the code of the Z340, the Zodiac killer’s famous cipher.

Re-examining the evidence: Former LAPD homicide detective Sal LaBarbera, left, and cold-case expert Ken Mains, right, are trying to solve the murder once and for all

The ciphers, which were sent with letters to the police and newspapers in Northern California during the 1960s and 70s, contain letters and symbols that may hide clues as to the killer’s identity.

Part of the research includes giving the supercomputer tasks like writing poetry with the mind of the Zodiac.

Mains and LaBarbera said a chance of catching the Zodiac was an opportunity they couldn't turn down.

LaBarbera said:'For me being a homicide detective for over 28 years it's just one of those fascinating cases that is kind of in the back of your mind.

'I've handled so many of my own cases and that was kind of my problem, but you always are interested in these old serial killer cases that are yet to be solved, and I was presented with the idea of attacking probably one of the most infamous unsolved cases in the United States. I thought it was a great opportunity.'

Who was the Zodiac Killer? A University of Southern California professor created an artificial intelligence software that was designed to help crack the code of the Z340, the Zodiac killer’s famous cipher (above) Although the Zodiac Killer claimed to have killed as many as 37 people in his letters to local newspapers, police have only linked five murders - and two other injuries - to him. On December 20, 1968, David Arthur Faraday, 17, and Betty Lou Jensen, 16, were on their first date when they pulled over into a lovers' lane on Lake Herman Road in Benicia. There, they were forced from the car by a killer and Faraday was shot in the head. Jensen began to run away but was shot multiple times in the back. On July 4, 1969, Michael Renault Mageau, 19, and Darlene Elizabeth Ferrin, 22, were in the parking lot of Blue Rock Springs Park in Vallejo - 4 miles from the first crime scene - when they were shot 'to pieces', according to the investigator. But while Ferrin was pronounced dead on arrival, Mageau survived being shot in the face, neck and chest. On September 27, 1969, Bryan Calvin Hartnell, 20, and Cecelia Ann Shepard, 22, were having a picnic at Lake Berryessa in Napa County when they were approached by a hooded man who bound and stabbed them. Hartnell survived eight stab wounds to his back, but Shepard died two days later. On October 11, 1969, cab driver Paul Lee Stine, 29, picked up a passenger and drove him to Presidio Heights - where he was shot in the back of the head and robbed. The killer also ripped part of Stine's shirt, which he later sent with a letter to a local newspaper. Four other victims and one other escapee have been connected to the Zodiac Killer, but none have been confirmed. Advertisement

Zodiac Killer: Case Closed? aired on HISTORY UK on Wednesday 22 November