LOS ANGELES -- The defense of O.J. Simpson could hinge on one thing: DNA testing that is so foolproof that it has helped put hundreds of suspects behind bars.
If final DNA results of blood samples at the crime scene don't match Simpson's, it would bolster defense lawyer Robert Shapiro's claims that Simpson was nowhere near his ex-wife's condo when she and a friend were brutally stabbed on June 12.
"That would be very dramatic," says Laurence Tribe, Harvard University law professor. "All bets would be off."
But if they match, denying that Simpson was there would be nearly impossible.
"An expert witness can testify that it means it's him or her to the exclusion of anyone else in the world," says James E. Starrs, professor of forensic science at George Washington University.
Police investigators are not confirming reports that they've already matched blood at the crime scene to Simpson's blood type and that early results of DNA tests, which match genetic codes, also provide a match.
The strongest DNA evidence available - Restriction Fragment Length Polymorphism - is still weeks away.
From a spot of blood the size of a quarter, an RFLP test can be as compelling in a courtroom as fingerprint evidence.
The most conclusive results come from four or five probes of the blood's genetic makeup. Each test rules out hundreds, then thousands, then millions of people.
"It can be as conclusive as millions and billions, depending on the statistical data," says Rich Cunningham, a forensic scientist with Lifecodes Corp., a private DNA laboratory in Stamford, Conn.
Another piece of strong evidence would be flesh samples from under the victims' fingernails, which could undergo the same type of DNA testing. Investigators haven't said whether they have such evidence.
Each DNA probe takes a week, and it takes one to two more weeks to analyze tests - a total of six to 12 weeks.
Los Angeles police won't disclose who's doing the testing. But Starrs suggests that in a case of this magnitude, police might request testing from two independent labs. About two dozen labs are capable of DNA testing - including one at the FBI.
In 1990, the FBI began to develop a national index of DNA records of convicted offenders. A year later, the index helped solve a case for the first time.
Semen was found at the scene of a 1991 rape and murder in Minneapolis. A DNA profile from the unknown suspect was searched against DNA records. A single DNA association was discovered and additional analysis confirmed the match. A suspect was identified, arrested and convicted.
A positive DNA match could throw Simpson's current defense out the window.
"What you're hearing Bob Shapiro saying is what any defense lawyer would say," says Barry Levin, a criminal defense lawyer and former Los Angeles police officer. "He's not guilty, not guilty until proven otherwise, and he's saying: 'Prove it.' "
The DNA test could do just that for prosecutors and could force Shapiro to change gears - fast.
In that case, Tribe expects Shapiro to argue that Simpson was there but that the crime was not premeditated.
"He intended to go there, frighten her and unexpectedly found her with this younger guy and lost it," Tribe offers as a possible defense scenario. "He premeditated something but not this (slaying)."
As crucial as forensic evidence may be, Levin warns that prosecutors often rely on it too heavily and bore the jury.
"Putting on a scientific forensic case is very monotonous; it's very slow," he says. "And when you're all done, the defendant gets up and says, 'Hey, I was there, but . . .' "
That's when the jury gets caught up in the emotions of a "heat of passion" or "diminished capacity" defense.
Take the Menendez brothers case, where the prosecution spent much time dwelling on premeditation. The brothers had already confessed to killing their parents, and their defense focused on motive - years of emotional and sexual abuse. Result: hung juries.
Says Levin: "Prosecutors generally are really, really good at showing what happened but lousy at showing why it happened."
