It seemed incriminating when Clemens' longtime personal trainer, Brian McNamee, came forward last week with needles, gauze and vials - allegedly used to inject the star pitcher with steroids and human growth hormone.

Experts say that while DNA technology is nearly perfect at matching samples to suspects, it can't necessarily connect Clemens to the drugs he's accused of using.

For one thing, the way the evidence was handled makes it hard to discount the prospect of tampering. "In forensic science, it all comes down to chain of custody," said Heather Coyle, an assistant professor of forensics at the University of New Haven.

Normally, evidence should be seized from a crime scene, securely collected and taped, dated and initialed, then stored in a place where nobody could tamper with it.

But in this case, McNamee, a former police officer, claimed he injected Clemens 16 times from 1998 to 2001, then saved the needles and syringes himself until early this year. His lawyers said he turned them in to federal prosecutors Jan. 7. McNamee also turned in some vials, gauze and a beer can that he said was used to store used needles.

DNA can last for years and still be useful to solve cold cases, Coyle said. Investigators can check it for 13 markers - spots on the genetic code that vary from person to person.

The only way

Needles and syringes could be switched, so it would help if they could find both substances on one or the other, the experts said.

They're unlikely to find Clemens' blood and steroid traces in the syringes, said Howard Harris, a forensic researcher at the University of New Haven. When something is injected into a vein, some blood comes back into the syringe, he said. But steroids aren't given that way.

"They're usually given intramuscularly, meaning they shoot them in the butt," he said. "There would not likely be any blood in the syringe."

The only way they might build a case is if someone can find Clemens' DNA and traces of illegal drugs on the same needle.

Laboratories can detect minuscule traces of almost anything with an instrument called a gas chromatograph.

It can separate a mixture into its component parts by measuring how fast they vaporize and move through a tube. Then a mass spectrometer can identify them - creating unique signatures that can be matched to a massive database of almost every compound known to man.

'No guarantee'

So the science is established to identify even the tiniest 10-year-old traces of DNA and steroids, but the problem is in the handling and nature of the evidence.

"In court you'd have a lot of trouble getting this admitted because there's no guarantee this particular syringe is connected to him," said Harris.

"The question is about the needle - where has it been and who had access to it and why did they have it."

The DNA evidence may prove less incriminating than reports of old MRI images that surfaced in Congress yesterday detailing an abscess or bruise in Clemens' buttock, possibly from botched steroid injections.

According to University of Pennsylvania physiology professor Lee Sweeney, scientists say that both steroids and human growth hormone can help athletes strengthen and build muscles and that, when taken together, they work synergistically.

"There's no doubt that it works, Sweeney said.