CrimeWatch

SAN FRANCISCO (CBS) ― There's a major roadblock in the Federal Bureau of Investigation's probe into the legendary Zodiac case. It's a lack of cooperation, not from a person, but from the San Francisco Police Department, FBI sources said. 

The FBI began collecting DNA evidence last year, but the SFPD - which is the agency that has most of the original evidence - isn't sharing. There may also be another roadblock, sources said. The FBI has in their possession a copy of a report alleging someone made changes to what are supposed to be authentic Zodiac letters.

The changes are subtle.

The Zodiac had seven victims in Northern California. In numerous letters, the psychotic killer revealed details of the attacks, taunted police and threatened to carry out more murders.

Dennis Kaufman claims his efforts to prove his late step father, Jack Tarrance, is the Zodiac uncovered changes to what are supposed to be copies of authentic Zodiac writings.

"They are different significantly," said Kaufman.

The theory is now capturing the interest of the FBI, whose agents are looking at a report complied by Nanette Barto.

Barto is a forensic document examiner. She believes she identified dozens of similarities in the writings of Jack Tarrance and the Zodiac, and discovered the changes.

Barto believes an entire Zodiac letter was painstakingly traced.

"At the bottom of each of these letters, where you see these heavy little dots, indicators of somebody writing slow and coming to a stop, almost as if they didn't want to overshoot it or add anything extra to it," explained Barto.

The tracing, Barto believes, is an effort to make the ink throughout the letter consistent, trying to hide a single alteration.

"I believe the 'a' in watch was altered, this previously had a bell top 'a'," said Barto.

The lines blend together, but when compared to an older copy, the bell top or hood over the 'a' appears to have been removed.

Barto claims to have uncovered several cases of altered a's.

Changing a single letter may not appear significant, but in the world of handwriting analysis it can be crucial.

"One missing similarity can outweigh all similarities found," according to Barto. Meaning, Jack's Tarrance's a's were so distinctive, most containing that extra line above that never finding one could create doubt.

When asked if it looked like someone intentionally altered the letters, Bart replied, "Absolutely."

Dennis claimed the changes began to appear after he showed his stepfathers handwriting to the San Francisco police department in 2001, and posted samples on the internet.

Who made the changes is unknown, but where they showed up is clear.

"We know in Graysmith's books they've been altered as well as Tom Voight's website," claimed Kaufman.

An author and a webmaster, Tom Voight runs Zodiackiller.com. The site said it is committed to archiving all case related material.

The changes are also found in print.

Robert Graysmith wrote two books, 'Zodiac' and 'Zodiac Unmasked.' The books were the basis for the 2007 movie "Zodiac."

Both books contain the same Zodiac letter, but are clearly not the same. Numerous markings were removed.

The text could have been cleaned up to make the letter easier to read, but why change the 'a'? Was it intentional or perhaps improvements in the quality of the copies?

Calls and e-mails to inquire which is the true text to Voight did not get a response. Attemps to contact Greysmith were also unsuccessful.

Handwriting is one piece of a much large puzzle the FBI is assembling to determine if Jack Tarrance and the Zodiac are one in the same.

Last August, the agency began running DNA tests on multiple items turned over by Dennis Kaufman, including a hood similar to one worn by the Zodiac.

If handwriting plays an important role, why wouldn't the FBI go straight to the originals? 

Because some in the FBI claim the SFPD is slowing down the FBI investigation.

The FBI has been asking since the year 2000, nine years, to view the police department's evidence, so far they maintain they have been shut out.

Only one Zodiac murder was committed in San Francisco, but the department has the majority of the Zodiac letters and the only partial DNA profile of the killer.

The SFPD declined to respond to multiple requests specifically asking if they would allow the FBI to examine their evidence.

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