RAMALLAH, West Bank — Yasser Arafat’s mysterious death in 2004 turned into a whodunit Thursday after Swiss scientists who examined his remains said the Palestinian leader was probably poisoned with radioactive polonium.
Yet hard proof remains elusive, and nine years on, tracking down anyone who might have slipped minuscule amounts of the lethal substance into Arafat’s food or drink could be difficult.
A new investigation could also prove embarrassing — and not just for Israel, which the Palestinians have long accused of poisoning their leader and which Israel has denied.
The Palestinians themselves could come under renewed scrutiny, since Arafat was holed up in his Israeli-besieged West Bank compound in the months before his death, surrounded by advisers, staff, and bodyguards.
Arafat died at a French military hospital on Nov. 11, 2004, at age 75, a month after suddenly falling violently ill at his compound. 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.
The Swiss scientists said that they found elevated traces of polonium-210 and lead in Arafat’s remains that could not have occurred naturally, and that the time frame of his illness and death was consistent with poisoning from ingesting polonium.
‘‘Our results reasonably support the poisoning theory,’’ Francois Bochud, director of Switzerland’s Institute of Radiation Physics, which carried out the investigation, said at a news conference.
Bochud and Patrice Mangin, director of the Lausanne University Hospital’s forensics center, said they tested and ruled out innocent explanations, such as accidental poisoning.
‘‘I think we can eliminate this possibility because, as you can imagine, you cannot find polonium everywhere. It’s a very rare toxic substance,’’ Mangin said.
Palestinian officials, including Arafat’s successor, Palestinian President Mahmoud Abbas, had no comment on the substance of the report but promised a continued investigation.
The findings are certain to revive Palestinian allegations against Israel, a nuclear power. Polonium can be a byproduct of the chemical processing of uranium, but usually is made artificially in a nuclear reactor or a particle accelerator.
Arafat’s widow, Suha, called on the Palestinian leadership to seek justice for her husband, saying, ‘‘It’s clear this is a crime.’’
Speaking by phone from the Qatari capital Doha, she did not mention Israel but argued that only countries with nuclear capabilities have access to polonium.
Israel has repeatedly denied a role in Arafat’s death and did so again Thursday. Paul Hirschson, a Foreign Ministry official, dismissed the claim as ‘‘hogwash.’’
‘‘We couldn’t be bothered to’’ kill him, Hirschson said.