Paris, France - The Swiss scientists who found "moderate" support for the theory that Yasser Arafat was poisoned with the radioactive element polonium 210 have challenged findings by French investigators.

In the French report, which has not been made public but was reviewed by the Swiss, investigators reportedly found similar levels of polonium 210 as the Swiss did.

While the Swiss suggested that these heightened levels may provide moderate support for the theory that Arafat was poisoned, the French said the polonium levels were a result of naturally occurring radon gas in Arafat’s tomb and that the Palestinian leader died of natural causes.

Polonium 210 exists naturally in low levels, formed from the decay of radon gas present in soil and in the atmosphere. It can also be artificially created in a nuclear reactor. In sealed environments, such as tombs, the accumulation of radon gas can produce slightly higher levels of polonium.

On Thursday afternoon, Francois Bochud, director of the Institute of Radiophysics in Lausanne, gave an interview to Al Jazeera in which he said that the French had found similar levels of polonium 210 in Arafat’s remains. However, he noted that his team, unlike the French one, took radon measurements from the tomb.

"For us, radon could be ruled out because actually we did measure radon in the tomb before opening it, and the values we found were about the same as we would find in any tomb," Bochud said. "Actually, it was a bit lower than what we could expect in normal soil. For us, radon is really an explanation that cannot be used."

French investigators did not respond to Al Jazeera’s request for comment.

In an interview on Wednesday evening with Radio Television Suisse, Bochud’s colleague, Patrice Mangin, director of the University of Legal Medicine in Lausanne, said he was "surprised" that French investigators ruled out polonium poisoning as a possible explanation for the Palestinian leader’s death.

"I want to remind you that Mr Arafat was hospitalised for two weeks in Paris," Mangin said. "The best experts were on his case, and no cause was established. And notably, the theory of infection was easily rejected. No fever, no infected area. All microbiological tests came back negative.

"It seems strange to me because the moment you rule it a death of natural causes that means you have a precise diagnosis," Mangin continued. "The appearance of such an advanced diagnostic seems debatable to me."

In July 2012, Al Jazeera’s documentary What Killed Arafat? reported how the same Swiss scientists found elevated levels of polonium 210, one of the element’s isotopes, in blood, sweat and urine stains on Arafat’s clothes.

That same month, Suha Arafat, the widow of the late Palestinian leader, filed a murder complaint at a criminal court in Nanterre, a western suburb of Paris, on July 31, 2012.

Based on the Swiss analysis of Arafat’s belongings that suggested high levels of polonium, a French judge appointed three prosecutors to investigate, resulting in a French forensic team being sent to Ramallah to collect samples during an exhumation. The team from Switzerland, as well as a new Russian team invited by the Palestinian Authority, also collected samples for analysis.

The resulting Swiss analysis found at least 18 times the normal levels of polonium 210 in Arafat’s remains. In contrast, the Russian study concluded that the cause of death by high levels of polonium penetration was "unsubstantiated."

During his interview with Al Jazeera, Bochud said he was unsure whether questions surrounding Arafat’s death could ever be answered.

"For us, I think we did really our best to take as much information from the samples we collected," Bochud said. "But maybe there will still be a mystery afterward."