But when her findings in a brutal double killing in Pasadena didn't conform to investigators' assumption about who committed the crime, Johnson suddenly found her expertise being called into question. In fact, at least two law agencies -- the Sheriff's Department and the Pasadena Police Department -- stopped utilizing Johnson's services early this year after the District Attorney's Office launched a review of her work procedures. While awaiting the results of that review, both departments have been contracting with more expensive private labs for DNA testing.
Johnson's troubles began in April of last year with the killings of sisters Martha Durrett and Linda Harrison, who were bludgeoned to death inside the Pasadena home they shared. Joe Durrett, Martha Durrett's estranged husband, quickly became the prime suspect for Pasadena homicide detectives, who realized early on that they would need extensive forensic evidence to pin the murders on him. One week after the killings, the investigators received the evidence they had been hoping for, and shortly thereafter, Joe Durrett was charged with two counts of capital murder. He was arrested at Ben Taub Hospital, where he was recuperating from wounds he had sustained two days after the murders in a mysterious shooting outside his Houston home.
The charges against Durrett were based on the results of tests by the ME's Office on strands of hair found in a bloody clump in one of Martha Durrett's hands. The medical examiner's forensic hair analyst determined that some of that hair matched samples of Durrett's hair. By combining those test results with reports that Durrett had been harassing the women before the murders, detectives were confident they had built a strong case.
But after Elizabeth Johnson took a closer look at that forensic evidence, the case against Joe Durrett began to unravel.
When evidence from the murders of Martha Durrett and Linda Harrison was brought to the ME's crime lab, Johnson was in California testifying in an evidentiary hearing on behalf of the Santa Cruz County district attorney. The California judiciary traditionally has been reluctant to accept new scientific techniques in criminal investigations, and Johnson was in court to explain a DNA protocol she and a colleague had recently developed.
A polymerase chain reaction -- or PCR -- is one of two types of DNA analyses. PCR, which is used when there is not much DNA available for testing, is not new, but Johnson claims to have developed a technique for taking the process a step further.
"It's a pretty simple idea," she says. "It takes a pre-existing system. You modify that to add in a set of DNA primers that will target an additional DNA region. It improves the sensitivity. So instead of getting just one bit of information, you get two."
That second bit of information, Johnson explains, will reveal whether the donor of the DNA sample is a man or a woman. Additionally, she says, with her protocol, test results can be obtained from much smaller samples of DNA.
Both of the added benefits from Johnson's protocol played a significant role in the Santa Cruz County Sheriff's Department's investigation of a 1993 murder of a woman who had been dismembered and whose body parts were stuffed into garbage bags and tossed into Monterey Bay. A co-worker of a man arrested in the killing told investigators that he had witnessed the suspect throwing a hacksaw from a truck window on the day of the murder. The hacksaw was found, and a California state crime lab found a small amount of blood on the blade -- but not enough to determine if the blood was that of the victim.
Assistant district attorney Mike Bartrum had heard of Johnson's gender testing technique and forwarded blood samples from his investigation to Harris County's DNA lab. "She tested our sample and it tested positive for female blood," says Bartrum. "That was introduced in our trial and was very significant in its outcome."
The suspect was convicted of the woman's murder. Bartrum says it's hard for him to imagine anyone -- especially prosecutors -- questioning Johnson's methods.
"Her credentials are flawless," he says.
After returning from California, Johnson began conducting DNA tests on the blood samples from the double murder in Pasadena. What she discovered over the next few months surprised her.