Genealogists have used DNA technology for years to identify unknown relatives, and law-enforcement officials have used the technology to identify criminals.
Now those two threads have merged. Today, investigators regularly use partial DNA matches to track down criminal suspects through family members who are already in a DNA databank.
Tracking Down BTK
Wichita District Attorney Nola Foulston spent almost 20 years chasing the serial killer who called himself BTK, for "Bind, Torture, Kill." When she started in the 1970s, no one was using DNA evidence. But by 2005, when she felt she was closing in on her man, familial DNA connections were a key part of the investigator's toolkit.
Foulston had samples of the killer's DNA from crime scenes, and she had a strong suspicion that BTK was a man named Dennis Rader. She needed to know whether Rader's DNA matched the killer's. But police couldn't just walk up to Rader and ask him for a sample.
"They didn't have everything they needed at that point to take him into custody," Foulston says, "so you'd be leaving a guy out there with his DNA sample hanging out. And he was not inactive. He continued to plan homicides up until the day that we caught him."
The team learned that Rader's daughter had recently been in the hospital for a pap smear. Under a judge's order, the hospital gave investigators a sample of the daughter's DNA. In 24 hours, the results came back. It was a familial match. That's all police needed to pick up Dennis Rader.
They brought Rader to a special facility for interrogation. After about an hour, one of the officers asked Rader, "You know why you're here?"
Rader replied, "I assume it's about BTK."
The officer asked, "Would you be surprised to know that the father of your daughter is BTK?"
"There was a stunning silence," Foulston says. "The FBI agent said, 'Tell us who you are.' And he said, 'I'm BTK. You got me.'"
Blind Sampling for a Partial Match
The Wichita investigators were lucky to have a specific target. They just needed a family member to validate their hunch. That's not how it usually works.
Typically, investigators will take DNA from a crime scene and compare it to all of the samples in a databank. They might find that the criminal is not an exact match with anybody on file, but the sample is so close to one in the databank that the criminal must be related somehow to the person on file. The question then becomes whether that is a legitimate investigative lead.
DNA technology consultant Chris Asplen used to direct the National Commission on the Future of DNA Evidence. He believes familial matches are as legitimate an investigative lead as a suspect's last name.
"Would you ignore that because you didn't have the person, but rather you had the person's brother?" Asplen asks. "Certainly not."
But civil libertarians don't think it is the same as a last name, because your name is public information. Your DNA profile is not. Tania Simoncelli of the American Civil Liberties Union says expanding DNA databanks and techniques such as familial searches "undermine the inherent notion that one should have privacy in their DNA."
Harvard professor David Lazer, a DNA expert, does not think that's reason enough for investigators to abandon familial searches.
"If we prohibited investigative processes that led to the investigation of innocent people," Lazer says, "we would just stop investigations altogether, because investigations always yield talking to and suspecting people who turn out not to be guilty."