Criminal Investigation Is Just a Human Art

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Franklin E. Zimring is law professor and director of the Earl Warren Legal Institute, UC Berkeley

The sad part of last week’s events in Yosemite was that it took one more killing than might have been to solve the multiple murders that had cast a pall over California’s most famous natural wonder.

Authorities had questioned their current suspect in the February killings, but they were not looking for a lone offender and there was no evidence that Cary Stayner was involved with potential co-offenders. Stayner also had no serious criminal history, a red flag for police in stranger violence. With little physical evidence to sort the innocent from the guilty, investigators must rely on their theories of how a crime happened to separate the high-priority suspects from the hundreds of possible offenders that any major investigation of a stranger-killing will generate. This time, the theory was wrong.

But was the Yosemite investigation bad luck or bad police work? It is both natural and necessary for experienced criminal investigators to adopt a working theory of how a killing occurred and to evaluate possible suspects and leads by measuring them against the official theory of the case. But constructing such probabilities is as much art as science, and last week’s lesson is that mistakes can be fatal.


How can it be that trial and error is still the best police practice in an age of DNA testing and supercomputers? Part of the answer to this question is the difference between a police investigation to identify unknown suspects and the use of physical evidence in criminal prosecution. The forensic laboratory is only occasionally successful in identifying a criminal suspect not already targeted. Sometimes a fingerprint on record will be the law’s first contact with the suspect. But DNA and tissue samples can only match a suspect when authorities have good biological samples from a crime scene and can get samples from potential suspects. And good lab work takes time and money in large measure.

At the front end of a criminal investigation, the traditional police methods of interrogation, tips and informers are much more important investigative tools than the microscope. And when a violent stranger crime occurs, police work is accurately summed up in that famous line from the movie “Casablanca”: “Round up the usual suspects.” Computers can help print out the list of usual suspects, but that is a supporting role.

The forensic laboratory comes into its own after a suspect has been identified and the question shifts to establishing or disproving guilt. DNA matches and blood evidence can be an important part of establishing guilt or innocence with a significant degree of science. Even here, however, there is a large range in the margin of error between the different branches of forensic science. Fiber analysis and handwriting analysis are far less exact than blood and fingerprints, even among the best practitioners. And the O.J. Simpson trial reminds us that sloppy lab work can deliver a crushing blow to the scientific rigor of any physical evidence-gathering process.

For the most part, good police work will remain methodical and low-tech work for the foreseeable future.

One old-fashioned lesson of the Yosemite murders is that authorities should never put total faith in any single theory of a crime. An open mind to how a crime may have happened is one important law enforcement tool.

Good police practice is not error-free detection; it is the ability to learn quickly from mistakes and not repeat them. One famous example of a dangerously exclusive law enforcement theory comes from the 1970s. The first time the U.S. Secret Service interviewed a suspect just before an assassination attempt on a president, the suspect was released because, being female, she did not fit the profile of an assassin. Shortly after that interview, Sara Jane Moore changed that profile for all time. And the very next dangerous attacker on a president, Lynette “Squeaky” Fromme, was also female.


Another lesson of the Yosemite killings is that humility also should be an important law enforcement tool. Supposing that the first three Yosemite murders were the work of more than one offender was wrong, but by no means was this mistake bad police work. Yet announcing that the killers were in custody when the evidence of guilt was quite weak is unprofessional in the extreme.

The problem in the Yosemite investigation was hubris rather than bad theory. The police had settled on suspects who had long police records, but there was little to tie those particular bad guys to the murders. The more confident the authorities are in the guilt of their current suspects, the less receptive they will be to follow alternative leads. When police round up the usual suspects in stranger-violence cases, they often will find the culprit, but not always. The best detectives also must keep a healthy respect for the unknown and the unpredictable.