IN less than a fortnight, the Tripoli Six will learn if they are to be executed. The medics – five Bulgarian nurses and a Palestinian doctor – have been imprisoned in Libya for seven years, facing the death sentence, for a crime they say they did not commit.

In 1999, they were accused of deliberately infecting 426 Libyan children in hospital with HIV, in collaboration with the CIA. Their "confessions" were extracted after a period of torture which included, say eyewitnesses, beatings with wire, electric shocks and the use of dogs. Ever since, they have protested their innocence, backed by an international coalition of medics, virologists, politicians and human-rights groups.

At their trial, one of the world's greatest experts on HIV, Dr Luc Montagnier, testified that the real cause of the infections was poor hygienic practices at the hospital. Records suggest the outbreak began before the medics arrived, and continued to spread after they were thrown in jail. And almost half the HIV-infected children were also infected with hepatitis B or C, which again suggests poor hygienic practice. But Montagnier's evidence was thrown out by the courts, and in 2004 the six were sentenced to death by firing squad.

After an international outcry, a retrial was granted by the Libyan Supreme Court. This trial ended recently, with defence lawyers still fearing the worst.

In desperation, human rights organisations launched a last-ditch appeal to scientists around the world to find definitive evidence that would exonerate the group. Today, with just 12 days until the verdict, that call has been answered – thanks, largely, to the ingenuity of an Edinburgh scientist.

Dr Andrew Rambaut had only just arrived in his new office at the Institute of Evolutionary Biology in October when the telephone rang. It was Professor Tulio de Oliveira, Rambaut's former colleague at Oxford University, who had been asked to head up a coalition of international experts to provide the missing evidence: the precise date of the HIV outbreak in the Libyan hospital, a secret hidden within the DNA of the virus itself.

A group of Italian scientists had gained access to new blood samples collected from the HIV-infected children. They needed a powerful, sophisticated computer program that could find the clues in the virus and interpret them – and such a program had already been written by Rambaut. "It's fair to say it was urgent," says Rambaut. "It went right to the top of my list."

He set out to date the outbreak using a technique similar to that used by taxonomists to date the evolution of a species. His program began comparing HIV samples taken from 51 of the 426 children. Each child had a slightly different lineage of virus, but each lineage had evolved from a common ancestor – a single strain of virus with a single DNA sequence.

Over two fraught weeks, the program was able to determine the original virus sequence, and reconstruct the exact phylogeny, or family tree, of the outbreak. Each new branch in such a tree is due to a mutation – a tiny change in the virus DNA sequence as the virus transmits from person to person.

Now, here comes the clever part. The rate of these mutations is almost constant – so regular that you can mark time by it. Thus, by beginning at the children's HIV sequences and travelling backwards through the branches, the computer was able to count the mutations and thus determine the length of time that had passed since the first infection in the hospital.

It showed that the strain of HIV with which the children had been infected was a West African subtype, which had clearly been present and spreading locally in the mid-1990s, long before the medics arrived. If the "molecular clock" was stopped on the day they arrived, in March 1998, it would show that the original virus had already branched into many, many different viruses – a sign that it had already been transmitted many times between people within the hospital.

The findings, published today in the journal Nature, provide independent genetic confirmation of Montagnier's testimony. The journal's editors have fast-tracked publication to make this new evidence available before the verdict – but whether that will be enough to save the Tripoli Six from the firing squad is a question no computer program can answer.

"It's not my place to comment on the outcome of the case," says Rambaut. "But I'm hopeful that something will come of this."


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