A Russian probe into the demise of former Palestinian leader Yasser Arafat has found that his death wasn't caused by radiation — a finding that comes after a French probe found traces of the radioactive isotope polonium and a Swiss investigation said the timeframe of his illness and death was consistent with that of polonium poisoning.

Vladimir Uiba, the head of the Federal Medical and Biological Agency, said Thursday that Arafat died of natural causes and the agency had no plans to conduct further tests.

Teams of scientists from France, Switzerland and Russia were asked to determine whether polonium, a rare and extremely lethal substance, played a role in Arafat's death in a French military hospital in 2004.

French experts found traces of polonium but said it was "of natural environmental origin," according to Arafat's widow, Suha Arafat.

Swiss scientists, meanwhile, said they found elevated traces of polonium-210 and lead, and that the timeframe of Arafat's illness and death was consistent with poisoning from ingesting polonium.

"It was a natural death. There was no impact of radiation," Uiba said, according to Russian news agencies.

In October, he was quoted by the Interfax news agency as saying that Arafat "could not have been poisoned by polonium" and that "traces of such a substance were not found."

There was no immediate explanation as to how the three investigations could have come up with different conclusions.

Remains exhumed

Palestinians have long suspected Israel of poisoning Arafat, which Israel denies. Russia, meanwhile, has had strong ties with Palestinian leaders since Soviet times, when Moscow supported their struggle.

Dr. Abdullah Bashir, the head of the Palestinian medical committee investigating Arafat's death, said they were studying the Russian and Swiss reports.

Arafat's widow filed a legal complaint in France seeking an investigation into whether he was murdered after a 2012 report which said traces of polonium were found on his clothes.

As part of that probe, French investigators had Arafat's remains exhumed and ordered a series of tests on them.

Arafat died Nov. 11, 2004, a month after falling ill at his West Bank headquarters. At the time, French doctors said he died of a stroke and had a blood-clotting problem, but records were inconclusive about what caused that condition.

Polonium occurs naturally in very low concentrations in the Earth's crust and also is produced artificially in nuclear reactors. There are also tiny, generally undetectable amounts of polonium in humans.  

The element can be a byproduct of the chemical processing of uranium, but usually it's made artificially in a nuclear reactor or a particle accelerator. Dozens of countries including Russia, Israel and the U.S. have the nuclear capability to produce polonium.

One of the most famous and recent cases of polonium-210 poisoning was that of ex-KGB officer Alexander Litvinenko, who died in London in 2006 after drinking tea laced with the radioactive isotope.

Britain has accused two Russians of the 2006 killing, but Moscow has refused to extradite them.