May 2, 2011 -- An analysis of DNA from Osama bin Laden's relatives that federal authorities said they relied on to confirm his death with 99.9 percent certainty is identical to a paternity test that determines whether a father and child are related.
"As humans, all of us are 99.9 percent identical. It's that 0.1 percent variability that distinguishes us, and that's what DNA testing looks at," said Dr. Ronald Crystal, chairman of genetic medicine at New York Presbyterian Hospital/Weill Cornell Medical Center in New York City.
DNA collected from bin Laden's body Sunday was compared to DNA from multiple relatives, a U.S. intelligence official told ABC News. One sample was analyzed, and genetic information was transmitted electronically from Bagram, Afghanistan, to Washington, D.C. Another sample will be physically transported to the United States for analysis.
It is unclear whether bin Laden's sister, who died of brain cancer in Boston in 2005, was one of the relatives used in the comparison.
When performed by trained technicians, the DNA-matching technique is almost 100 percent accurate.
"The chances that they're a match are probably 99.999 percent," Crystal said. "The probability that two unrelated DNAs would match so closely is extremely small, but you can never be 100 percent sure."
But the test can't rule out that the body was bin Laden's brother, according to Dr. Lawrence Kobilinsky, a forensic scientist and chairman of science at John Jay College of Criminal Justice in New York City.
"The best way to do it is if you've got some of his own DNA [for comparison]," Kobilinsky said. "That would be the best situation."
Rep. Mike Rogers, chairman of the House Intelligence Committee and a former FBI agent, said multiple DNA samples from people related to bin Laden were used to confirm his identity "beyond a shadow of a doubt," the Associated Press reported.
DNA testing is used widely in the U.S. justice system to match DNA from a crime scene with that of a suspect's. The hearty molecule can persist for years but is easiest to compare when it comes from a fresh tissue sample.
"I assume they had fresh tissue or blood that they could test very easily," Crystal said of the military personnel who tested DNA from bin Laden, who was shot dead in Abbottabad, Pakistan, Sunday. "DNA testing has become very sophisticated. You can even evaluate DNA from extinct animals or humans that have been dead for many years. It's pretty tough stuff."
Details of the technique used to match bin Laden's DNA against his relatives' are unknown. Usually, however, the DNA is cut into small pieces and forced by an electrical current through a gelatin-like substance, where it separates into horizontal stripes according to size. The stripes form a ladder-like pattern that's specific to an individual, like a fingerprint.
"Mother and father DNA are shuffled together, so you get pieces from mom and from dad and that's what make you an individual," Crystal said. "If we share a mother and father, the chances that we have similar DNA are much greater than if we have different parents."
Despite the 0.01 percent uncertainty, Crystal said, the 99.9 percent match is "absolutely" enough to be considered conclusive.
The al Qaeda leader was shot in the head after U.S. forces stormed the compound roughly 40 miles north of Pakistan's capital city of Islamabad. He was identified by two women in the compound and by members of U.S. military during the raid.
Facial recognition technology that revealed a 90-to-95 percent match between the corpse and photos of bin Laden reinforced the positive ID, officials said.
It is unclear whether officials will release a photo of his body.