It has been 20 years since radiocarbon tests concluded that the Shroud of Turin, which bears a mysterious image of a crucified man, was woven in the Middle Ages and thus couldn't be the burial cloth of Jesus, as many believed.

That was supposed to end the debate over the origin of the shroud, but it didn't.

What keeps the debate alive today is not just the hope of some faithful that the ancient cloth may be a relic of Jesus' death or resurrection, but also the insistence of many scientists and others caught up in the dispute that the puzzle pieces still don't add up to an object created in the Middle Ages.

They include not just Christians, but Jews and agnostics, and argue, among other things, that there were major problems with the carbon dating, including the possibility that the tiny swatch tested two decades was not part of the original shroud.

Tonight, Britain's BBC is airing a documentary questioning the carbon dating and detailing a new hypothesis that could explain how genuinely old linen could produce a much younger date in certain conditions

Scientists and archaeologists who spent years studying the 14-foot shroud agree that it bears the faint image, front and back, of a naked, beaten, crucified man in the grip of rigor mortis. Marks on his back and legs are consistent with a first century Roman whip. His wounds fit with gospel accounts of a crown of thorns and legs that were not broken as other crucifixion victims' were.

When the blood stains are viewed under a forensic light, a medically correct separation of serum from clotted blood is visible. Nail holes in the wrists resemble those of a crucifixion victim unearthed near Jerusalem in 1968, not the palm wounds found in medieval art.

The shroud's known history begins in the 1350s, when a knight brought it to France as spoils of a crusade. It has endured at least three fires -- most famously in 1523 when melted silver burned holes that were patched by nuns. It has been kept in the cathedral of Turin, Italy, since 1578.

The Catholic Church never declared it authentic. Bishops at the time were very alert to the trade in fraudulent relics. One denounced the shroud, saying he knew of someone who knew someone who knew someone who claimed to have painted it.

In 1898, the shroud was photographed, and when examined, it turned out the image was a negative, far more detailed on the reversed plate. In 1976, that inexplicable finding led two American rocket scientists to wonder what would happen if they put its picture under a VP-8 image analyzer, used to view pictures from space probes. These images do not record light, but the distance between the object and the camera.

Ordinary photos or paintings placed under the VP-8 are wildly distorted, with concave cheeks, twisted chins, ingrown hair. The shroud photo came back a perfectly proportioned, three-dimensional image.

One of those scientists, physicist John Jackson, recruited about 50 scientists and scholars from different disciplines, and obtained church permission to study the shroud. After three years of analysis, their consensus was that the image was formed by premature aging of the topmost fibrils in the linen thread, and the blood stains were human blood. But there was no agreement on what might have caused the image.

In 1988, church authorities sent a swatch from one corner to three laboratories for age testing. All three concluded that it had been woven from flax harvested around 1325, give or take 35 years.

Among those who argued most vigorously that the carbon results were invalid was archaeologist William Meacham, an honorary research fellow at the University of Hong Kong. He had advised the archbishop of Turin prior to the carbon-dating, warning that archaeological finds sometimes give "rogue" readings due to contamination or alteration from another era.

"The result is simply discarded and other samples dated. But in the case of the shroud, I knew that whatever the result, it would be pounced upon, which is why I argued so vigorously for taking at least two samples from different areas of the cloth," said Dr. Meacham, a Methodist.

"It was simply insane to take only one sample, from the corner near the seam. What is more infuriating, and at the same time incomprehensible, is why the church has not, over the last 20 years, taken another sample from another area for dating to confirm -- or otherwise -- the 1988 result."

The sample was very near a seam of unknown date that attached a border to one side of the shroud.

Last month, the London Telegraph reported that Christopher Bronk Ramsey, director of the Oxford University Radiocarbon Acceleration Unit, had said he was giving serious consideration to the theory that there may have been significant problems with the sample.

If there's any openness to reconsideration, Barrie Schwortz, a Squirrel Hill native who was team photographer for a 1978 scientific research project on the shroud, is glad to hear it. (His copyrighted photos are displayed on his Web site, www.shroud.com.)

He had his own change of mind. He is Jewish and, when he came to the project in 1977, "I was biased against it. I expected to see brush strokes and come home having had a free trip to Italy," he said.

Now, he said, "I feel obligated to the truth. Not to the man of the shroud, or to some religious ideal, but to the truth."

His favorite argument against the carbon date is an illuminated manuscript from 1191, called the Hungarian Pray Codex. It shows a detailed painting of a cloth that resembles the shroud, including a set of odd, L-shaped burn holes that are known to predate the 1532 burns.

"This artist had seen it at least 70 years earlier than the earliest date that the carbon dating said it could be from," he said.

In 2000, a theory emerged that the test site had been repaired in the Middle Ages using a technique called "French invisible reweaving."

Shroud researcher Ray Rogers, a specialist in the chemistry of burned materials, set out to prove that wrong. He had access to threads from a cut adjacent to the carbon sample, and compared them to other areas the team had studied.

His findings were published in the peer-reviewed journal Thermochimica Acta shortly before his death in 2005. The carbon area was covered with a gum-dye that might have been used to prevent raveling and to match the original color. There was also a substantial amount of cotton woven into it that was not found in other areas. The evidence supported a later repair to the shroud, he wrote.

"The thing about [Mr. Rogers'] theory is that it didn't find fault with the radiocarbon dating itself. It just said that the sample-taking was in error," Mr. Schwortz said.

Information critical of the shroud's authenticity has been compiled at www.skeptic.ws/shroud/ by micro-paleontologist Steven Schafersman.

The Web site puts forth the 2005 work of a French scientist who molded a damp linen sheet over a bas relief similar to the shroud image. He let it dry, then daubed it with a mixture of ferric oxide and gelatin.

The French fake "looked just like the shroud, but it lacked the chemistry and physics," Mr. Schwortz said, citing the forensically accurate blood stains and the three-dimensional data in the shroud.

The best-known skeptic was the late Walter McCrone, a microscopist who examined sticky-tape samples of debris from the shroud. He found the ancient pigment iron oxide, and argued until his death that the shroud was a painting.

But blood chemist Alan Adler, another Jewish member of the team who was involved in shroud studies until his death in 2000, ran highly sophisticated tests that he said proved the blood stains were human blood. There was iron oxide, but it wasn't concentrated on the image, his colleagues said.

Among those scientists who believe the shroud may be authentic, many, including Mr. Schwortz, Dr. Meacham and a Rome-based research group, expect it was produced by a natural process, not a miracle. One theory is that it resulted from browning caused by a chemical reaction between sweat, vapors from the body and impurities on the linen.

But Dr. Jackson, who with wife Rebecca now directs the Turin Shroud Research Center of Colorado, believes it requires a supernatural explanation.

"I think we are dealing with something very profound," he said.

"If the Shroud of Turin is authentic and it did wrap what we Christians believe is the Resurrection event itself, then we are dealing with an image that has radical characteristics."

That does not mean that his faith rests on the shroud or colors his findings, he said.

"I'm Catholic, and the center of my faith is the holy Eucharist," he said.

"The shroud never should be an object of belief or divine revelation. Only Jesus Christ himself is both."

Dr. Jackson, however, rejects the theory that the carbon date resulted from a medieval repair. That would have required microscopic work in an age before microscopes, he said.

He is working on his own theory about the carbon date, but isn't ready to discuss it. However, he sees no sign that the Archdiocese of Turin is open to further testing. He also shares the concerns of some shroud researchers that a 2002 restoration project may have damaged or removed scientific data on the shroud.

"We are very pleased that the radio carbon lab is open [to reconsideration]. There are no conclusions yet, but if they are open, we are open. If it works, great. If it doesn't work, that is the way of science," he said.