There's No Way bin Laden's Alive. How Do we Know? DNA

When President Obama made the announcement this week that U.S. Navy Seals had killed Osama bin Laden, the first question everyone asked was, How can he be so sure?

The answer: DNA.

As a criminal defense attorney, legal analyst and journalist who covered the innocence movement from its inception, I have long studied the DNA as a tool for exoneration, and identification.

So when I heard the government say it had a DNA match for bin Laden, I had a fundamental question: Match to what?

It was not that I doubted the time frame. I have covered the science of DNA and it’s use in the courts, for the last fifteen years, as DNA collection, testing and analysis evolved by leaps and bounds.

DNA analysis that once required weeks can now be performed in less than 24 hours. The process of extracting and purifying DNA can be done in less than an hour. Analysts need only a few hours to amplify, analyze and match the samples.

Which takes us back to the match: What was the reference sample on file for Osama bin Laden? In a typical case, involving rape or murder, a test sample of DNA from the crime scene would be compared with a reference sample already on file from the suspect to see if they matched. But this was not your run-of-the-mill case and this was not your typical suspect. Presumably, there was no reference sample on file for Osama bin Laden.

Bin Laden was unique in his status at the very top of the world's Most Wanted List; but he was just like the rest of us in relation to the human genome. His unique genetic code was shared with his parents and siblings. Think about it: Our parents are the ones who give us our DNA, to begin with. So the DNA swabs collected from the body in Pakistan would have been matched with reference samples previously collected from bin Laden's relatives.

Now it was starting to make sense.

It has been widely reported that the government collected DNA from bin Laden's half-sister, who died of brain cancer a year ago at Massachusetts General Hospital; after her death, government officials reportedly took some of her brain tissue for genetic testing.

So, that’s the DNA “match.” A close “match” identified Osama bin Laden with an extremely high probability of accuracy.

This is the famous (or infamous) 99.9 percent number, we hear so often in court. In the case of Osama bin Laden, the government is 99.9 percent sure the dead man is bin Laden because they compared his DNA to that of bin Laden's late half-sister. The results speak to the probability of a relationship between the two people, not the actual identity of the dead man.

To be 100 percent certain, the government would have had to compare Osama bin Laden to Osama bin Laden. And to do that, it would have needed a previously obtained bin Laden sample (say a hair plucked off his head by one of the two western journalists who interviewed him, or a fingernail clipping provided by an al Qaeda operative working with the CIA – unlikely scenarios, to say the least).

What we have instead is a DNA test comparing the profile of the man shot in Pakistan with a bin Laden family member. The result is a likelihood ratio. Taken together with bin Laden’s wife’s idenfication and photographic evidence, the proof was strong enough for President Obama to deliver his dramatic address to the nation late Sunday night.

Will we ever know for sure? 100 percent? Probably not, given that the body has been buried at sea. In some wild scenario, an original source reference sample could turn up. But I wouldn’t count on it. The government's 99.9 percent is good enough for the scientific community, and it's good enough for me. But if history is any guide, that one in one zillionth chance that Osama bin Laden is still alive will be just enough to satisfy conspiracy theorists for years to come.

Jami Floyd is an attorney, broadcast journalist and legal analyst for cable and network news, and is a frequent contributor to WNYC Radio. She is former advisor in the Clinton administration and served as a surrogate for the Obama campaign on legal and domestic policy issues. You can follow her on twitter.