A rape kit is a deceptively powerful tool, a cardboard box with envelopes, glass microscope slides, swabs, a comb and blank forms. As it journeys from hospital to police station to laboratory to court, the evidence within can lock up rapists, or free those arrested or convicted in error.

In the Duke Hospital emergency room on the morning of March 14, 2006, Dr. Julie Manly conducted a pelvic examination of Crystal Gail Mangum. Mangum said three men had gang-raped her for half an hour. The doctor also collected evidence: thong panties; swabs of the mouth, vagina and rectum; a cheek scraping; a blood sample; and hair from Mangum's head.

Manly made one physical finding: "diffuse edema of the vaginal walls," or generalized swelling. The doctor was assisted by Tara Levicy, a nurse in training to conduct rape exams.

The kit was passed from Levicy to Duke police to Durham police, who delivered it to the State Bureau of Investigation laboratory March 27.

The next day, agent Rachel Winn examined the swabs, slides and panties. She found no evidence of semen, blood or saliva. That ran against Mangum's accounts: She said that no condoms were used and that at least one man ejaculated, perhaps all three.

The following day, March 29, Durham District Attorney Mike Nifong started speculating publicly that the DNA test results might not match any of the lacrosse players. "I would not be surprised if condoms were used," he said.

It's unclear when Nifong learned that the rape kit had come up empty. On March 30, agent Jennifer Leyn called Nifong to discuss samples collected from the bathroom floor of the house where three lacrosse captains lived; SBI phone logs don't mention what else they discussed.

Nifong certainly knew by April 4 that the SBI found nothing in the rape kit; that's when agents from the lab updated him in a conference call. With no helpful evidence from the SBI, Nifong started looking for a laboratory that could perform more sensitive DNA tests.

Nifong found an eager scientist in Brian Meehan, director of DNA Security Inc. in Burlington. Meehan started his career as an academic biologist who specialized in studying mussels but gravitated to working in DNA laboratories in the 1990s, as DNA became a more powerful and sophisticated forensic tool.

An affable man with a strong Boston accent, Meehan opened his own lab in 1998. In recent years, he and his partners had aggressively marketed their lab. They conducted training sessions for police and public officials and offered private investigators discounts for multiple tests.

Meehan's lab could run tests so sensitive that they pick up DNA from a single cell.

Late on the afternoon of April 4, Meehan told Durham police he wanted in; Investigator Michele Soucie wrote that DNA Security "can possibly adjust prices because they would really like to be involved in case." Still, Meehan's work would eventually cost the state $28,610.

Nifong moved quickly. The next day, he got a judge's permission to perform the private tests, and the following day, April 6, a Durham police officer carried the evidence to Burlington, where it was tested April 7, 8 and 9.

On April 10, Nifong and his investigators went to Burlington to meet with the lab director. Nifong recently said he doesn't remember this meeting; however, Sgt. Mark Gottlieb and Investigator Benjamin Himan recorded it in their notes and narratives.

Meehan testified that he met Nifong on April 10 and told the prosecutor that he found no DNA of any of the players in the rape kit but that he found traces of DNA of unidentified men.

Jackie Brown, manager of Nifong's primary campaign, talked to the prosecutor before and after that meeting, when she called him to discuss the size and cost of a newspaper ad. She reached Nifong on his cell phone as he arrived at DNA Security.

Nifong was in a rare good mood and promised Brown he'd call her in a few minutes, after the meeting. Brown waited for Nifong to return the call. When Brown finally called him back, Nifong was gruff. He was passing through Hillsborough on his way back to Durham.

"His whole demeanor had changed. He was kind of ill and said, 'I really don't want to talk about the ad,' " Brown said. "Whatever happened, he wasn't happy with it."

Nifong had a big problem with his case. His chief witness said that three lacrosse players raped and sodomized her for 30 minutes without using condoms and that one or perhaps all ejaculated -- but if that happened, the men didn't leave a single cell behind.

The next morning, Nifong said at a locally televised forum at N.C. Central University that DNA didn't matter.

"DNA results can often be helpful, but, you know, I've been doing this for a long time, and for most of the years I've been doing this, we didn't have DNA. We had to deal with sexual assault cases the good old-fashioned way. Witnesses got on the stand and told what happened to them. ... It doesn't mean nothing happened. It just means nothing was left behind."

Despite the lack of a DNA link to the players, Nifong filed a sealed motion the next day saying he would seek indictments against Collin Finnerty and Reade Seligmann.

On April 21, Nifong returned to Burlington to meet with Meehan at DNA Security.

Nifong and Meehan discussed tests Meehan had run on the final samples from the rape kit. The results were the same: There was no DNA from the lacrosse players, but there was DNA from unidentified men. If Mangum had been raped, the DNA was a logical starting point to identify the perpetrators.

Nifong's chief assistant district attorney, David Saacks, had said so in the request for the judge's order that Nifong found on his office copier March 23:

"The DNA evidence requested will immediately rule out any innocent persons, and show conclusive evidence as to who the suspect(s) are in the alleged violent attack upon this victim."

Nifong and Meehan, however, agreed that Meehan would not report that the rape kit turned up DNA from at least four unidentified men. They knew that the DNA wasn't from lacrosse players, Mangum's boyfriend or one of Mangum's drivers.

Nifong then made an odd choice for someone directing a rape investigation:

He did not try to find out whose DNA it was.