Palestinian investigator Tawik Tirawi has said scientific reports show Yasser Arafat did not die from old age or ill health, nor did he die a natural death.

On Friday the Palestinian committee investigating Arafat's death, Tawfiq Tirawi, was giving its assessment of the research.

Earlier, the head of the committee said he had received the Swiss report last week along with the results of tests by the Russian team appointed by the Palestinian Authority.

Scientists from the Vaudois University Hospital Centre (CHUV) in Lausanne, Switzerland, had carried out a detailed examination of Arafat's medical records, samples from his remains and items he had taken into the hospital in Paris.

The biological materials included pieces of Mr Arafat's bones and soil samples from around his corpse.

Prof Francois Bochud told a news conference on Thursday that the high level of polonium detected "by definition... indicates third party involvement... Our results offer moderate backing for the theory of poisoning".

But he added: "Was polonium the cause of the death for certain? The answer is no, we cannot show categorically that hypothesis that the poisoning caused was this or that."

The scientists said they had been unable to reach a more definitive conclusion because of the time passed since Arafat's death, the limited samples available and the confused "chain of custody" of some of the specimens.

However, many Palestinians believe Israel was responsible for Arafat's death although Israel has repeatedly denied it.

Polonium-210 is a highly radioactive substance. It is found naturally in low doses in food and in the body, but can be fatal if ingested in high doses.

His official medical records say he died from a stroke resulting from a blood disorder.

Suha Arafat said she had no doubt her late husband had been assassinated but refused to point the finger at Israel.

"I can't accuse anybody. Everybody wants to accuse Israel - I can't accuse - I can't jump into conclusion," she told the BBC.

Arafat led the Palestine Liberation Organisation for 35 years and became the first president of the Palestinian Authority in 1996.