Special Reports

Trace of DNA played key role in cracking case

It was the smallest of clues that finally broke open a case that had been overwhelming in scope.

Detectives and forensic technicians used DNA - the very building blocks of human life - to tie Gary Leon Ridgway to one of the most prolific killing sprees in the history of Washington.

It is clear from court documents released Wednesday that without the traces of sperm Ridgway left on three victims 20 years ago, the man who has confessed to being the Green River Killer might still be at large.

"Despite the efforts of scores of detectives and forensic scientists, who received hundreds of items of physical evidence, and interviewed thousands of witnesses, the cases remained unsolved," according to a summary of evidence released Wednesday by King County prosecutors.

"None of the evidence appeared to link Ridgway with any particular scene or victim. However, one item of evidence seized would prove to be significant: a saliva sample taken from Ridgway during the execution of a warrant."

Forensic scientists, relying on new techniques developed in the past 10 years, used DNA from that saliva to tie Ridgway to the murders of Marcia Chapman, Opal Mills and Carol Christensen, a break that eventually blew the case wide open.

Ridgway, in an interview with a forensic psychiatrist after his arrest, summed it up this way: "What got me caught was technology."

DNA is the genetic material that governs everything from hair color to predisposition for diseases. Each individual has a unique DNA "fingerprint" that can be compared against samples obtained at crime scenes.

Based in large part on the DNA evidence, King County prosecutors charged Ridgway in 2001 with four of the nearly 50 murders blamed on the Green River Killer.

Once they had a suspect, detectives were able to connect him to three other murders using other physical and circumstantial evidence.

Those links, and the threat of execution, then led to discussions among Ridgway, his attorneys and investigators that culminated in Ridgway's confession Wednesday to 48 deaths, court documents state.

Detectives never would have made it that far without evolutions in forensic science.

In 1988, detectives with the Green River Task Force sent evidence associated with several victims to a private laboratory for DNA typing. Based on the technology available then, no matches were made.

Since then, scientists developed a new method of testing DNA called polymerase chain reaction, or PCR testing. In the process, DNA gathered by police can be replicated to increase the sample size, making it much easier to analyze.

In essence, small degraded samples like those in the Green River case could be used to try to make a match.

In 2001, King County detective Tom Jensen sent some DNA evidence in the Green River case to the state crime lab, hoping the new testing method might provide the clues the task force was looking for.

"This time a profile was developed," the summary of evidence states. "It matched Gary Leon Ridgway."

Adam Lynn: 253-597-8644