Investigators are gathering DNA evidence they hope will link suspected serial snipers John Allen Muhammad and John Lee Malvo to the Washington, D.C.-area shootings, ABCNEWS has learned.
So far police have been unable to find credible eyewitnesses to any of the shootings that left 10 people dead and three wounded in a three-week span, and it is unclear exactly who pulled the trigger. Because of these obstacles, investigators will have to rely on circumstantial evidence to link the two suspects to the crime scenes.
Sources told ABCNEWS that police have found what appears to be sweat or saliva residue on a package containing a letter demanding $10 million. The package was recovered near the scene of the Oct. 19 shooting of a 37-year-old man outside a Ponderosa restaurant in Ashland, Va.
"Of all the things that a prosecution's team can put before a jury, probably the most compelling thing in America in 2002 is DNA evidence," said former Deputy Attorney General Eric Holder.
The FBI is also planning to conduct DNA tests on what is believed to be human waste found in the woods near one of the shootings and on a raisin box found near another crime scene, sources told ABCNEWS.
According to criminal investigation experts, the federal crime labs at the Bureau of Alcohol, Tobacco and Firearms, FBI, and Secret Service will be using a mix of high-tech tricks and proven analysis techniques to help cement the case against Muhammad and Malvo.
"When you talk about state of the art [forensic] equipment, the labs are in good shape to look at the biologic and document evidence [in this case]," says Lawrence Kobilinsky, a professor of forensic science at John Jay College of Criminal Justice in New York. "The tools are all there."
A key part, obviously, has been the ballistic evidence — the few shell casings and bullet fragments recovered from the multiple crime scenes and victims.
"Ballistic investigation has been around for many years, but the capability has gotten better over the years," says Robert Castelli, a professor of criminal justice at Iona College in New Rochelle, N.Y., and at John Jay. "There are great gains from a technology standpoint. Just in the ability to have greater magnification in comparative microscopes is tremendous."
By examining recovered bullets and casing for similar markings, investigators were able to determine early on that most of the shooting cases were related to the same .223-caliber weapon. More importantly, investigators say they have been able to match that recovered ballistic evidence to the Bushmaster XM-15 rifle that was discovered in the suspects' car upon their capture.
But ballistics evidence is also helping investigators tie Muhammad and Malvo to other shootings as well.
Identifying Other Cases
Two other weapons recovered in the suspects' car have been positively identified with other shootings in Tacoma, Wash., where Muhammad was known to have lived. One of the weapons, a .45-caliber handgun, was used to kill Keenya Cook, the niece of a former employee at Muhammad's auto repair shop.
Outside forensic experts suspect that a computerized ATF database known as the Integrated Ballistic Identification System (IBIS) played a key role in that discovery. IBIS is a collection of digital images bullets and casings recovered by police nationwide to help investigators determine if evidence from one crime can be linked to others.
"If you get a bullet, it's routine procedure to take an image and compare it against all others in the [IBIS] database," says John Dillion, Jr., a firearms forensic consultant and retired FBI examiner familiar with the system. And since a fairly large number of shootings are committed by a few criminals, IBIS "is a really nice and elegant way of eliminating a vast number of possible [suspects]," says Dillion.
Still, at the end of the day, ballistics evidence can only tell investigators that the same weapon was used in all the cases. "Someone has to show who has the gun and [was] in possession of it when it was fired," says Dillion.
And for that, investigators will have to turn to other evidence that may be gathered from the piles of possibilities.
Picking Up Hidden Paper Clues
One other area that investigators will undoubtedly mine with great care is the tarot card and letter supposedly left at two of the sniping scenes.
"There's a number of evidence that you can gather from [those] documents," says Robert Kullman, a forensic document examiner with Speckin Forensic Laboratories in Okemos, Mich.
Handwriting analysis by federal investigators has already linked the two documents, saying the handwriting on both is identical. The next task, says Kullman, would be to match the writing with one of the suspects, from other writings found in the car or obtaining writing samples.
Investigators will also be trying to locate hidden fingerprints using lasers and chemicals. And they'll also look for even microscopic amounts of saliva, hair or other biological traces that could yield DNA evidence against the suspects.
"Just a few years ago, if I have a card and a suspect on it, would I be able to pick it up?" asks Kullman. "Probably not. But with technology now, you can get so much more out of a small amount."
Let’s Go to the Video
Still, the best possible evidence against the suspects would be a visual eyewitness.
However, since no credible eyewitnesses have come forward, investigators are also reviewing hours of surveillance video from areas around the crime scenes in hopes of catching a glimpse of Muhammad and Malvo. Experts say that since many of the attacks occurred in areas typically monitored by surveillance cameras, it's quite possible visual evidence of who pulled the trigger exists in miles of videotape.
"It's safe to say [federal investigators] are examining boxes of videotape," says Grant Fredericks, a video forensic evidence expert with Avid Technology who has trained investigators at the FBI headquarters in Quantico, Va. "The videotape could be a vital piece of collaborative evidence."
Fredericks says that investigators will key in on how the suspects got to and from each crime scene and look for surveillance cameras that might have caught the suspects or their Chevrolet Caprice on tape. The video can then be run through a digital editing system that would clean up the videos and help see if the car or suspect appear on the video, thereby placing them at the scene of the crime.
"There's nothing [the suspects] can do to prevent from being captured on video," says Fredericks. "It's the challenge [of the investigators] to get [the tape] and know what they're looking for."
Aside from the ballistics and document evidence, investigators and prosecutors have been mum about what evidence will be brought against Muhammad and Malvo during the trial. But just as law enforcement agents spared no effort in the manhunt for these suspects, investigators aren't holding back in finding the evidence that will close the serial sniper case for good. ABCNEWS' Pierre Thomas and ABCNEWS.com's Paul Eng contributed to this report.