CHICAGO – The suspected murder weapon is a radioactive substance found in nature and normally harmless, yet so toxic if swallowed that it can kill in doses smaller than a speck of dust.

The harmful particles it emits cannot pass through skin or through paper, making it relatively safe to deal with and relatively easy to conceal.

Yet if carelessly handled it can leave traces on surfaces it touches, and a contaminated person can excrete it through sweat or urine.

Eventually, the unusual properties of the radioactive element polonium-210 may be what allows authorities to track down who was responsible for the lethal attack on a former Russian KGB agent who died Nov. 23.

Amid international intrigue over the bizarre poisoning death of Alexander Litvinenko, the oddest development may be the substance that killed him. Experts in the field of health physics say there is no previous record of polonium-210 being used as a poison.

On Friday, British authorities said tests also had revealed polonium exposure in Italian security expert Mario Scaramella, who met Litvinenko at a London sushi bar on the day the former spy fell ill. Scaramella was hospitalized but has shown no symptoms of poisoning, a hospital spokesman said.

A friend of Litvinenko’s told The Associated Press on Friday that Litvinenko’s wife also was “very slightly contaminated” and did not need medical treatment.

The miniscule amounts of polonium used have set off an intense response by the British Health Protection Agency that one British paper described as similar to the planned protocols for a radioactive “dirty bomb.”

The investigation has revealed an invisible radioactive trail that includes a dozen locations around Britain and at least three jetliners that traveled between Britain and Moscow.

“Whoever did this, it doesn’t sound like they were neat,” said Don Cossairt, associate head of radiation protection at Fermilab in Batavia.

Although the polonium-210 isotope is safe to handle in a controlled environment, tiny bits of it scattered in public places would pose a risk, he said. “You don’t want people ingesting this stuff,” said Cossairt.

Polonium occurs naturally at low levels in ordinary soil and can be found in water, cigarettes and some vegetables. It has been used commercially in devices to eliminate static electricity. Most experts say it would take a nuclear reactor or particle accelerator to make significant quantities of the material; Cossairt said the accelerators at Fermilab do not make it in substantial amounts.

The isotope’s peculiar threat comes in the form of alpha particles, products of radioactive decay that are the same as what is found in the nucleus of a helium atom. Polonium-210 decays rapidly; the isotope has a half-life of just 138 days.

Its decay also releases intense heat if the substance is present in large amounts. One gram of the substance can reach a temperature of more than 900 degrees, which is a reason why the Russian space program used the isotope as an energy source for space probes.

The alpha particles that polonium releases cannot penetrate a sheet of paper, but if ingested they can kill cells by breaking strands of DNA, said Albert Wiley, director of a radiation emergency response center within the National Nuclear Security Administration.

“Before this incident I had never heard of polonium being used in a poisoning,” Wiley said.

One of the only previously suspected victims of polonium was scientist Irene Joliot-Curie, the daughter of researchers Marie and Pierre Curie, who were the co-discovers of polonium. Joliot-Curie, a Nobel Prize winner like both of her parents, died of leukemia 10 years after an accidental exposure to the substance.

When ingested, polonium-210 can damage the digestive tract and enter the bloodstream, where it can devastate a person’s bone marrow and vital organs. The isotope also can cause baldness because it damages hair follicles and any other tissue that undergoes a rapid turnover of cells, Wiley said.

Just one-millionth of a gram of polonium-210 could be a fatal dose, according to a fact sheet released by the Health Physics Society.

To sniff out the trail of the isotope, authorities are using instruments that can detect the alpha particles that polonium releases. The detection devices pick up particles by filtering the air or testing swipes taken from fabric.

Such searches have turned up signs of polonium-210 at Litvinenko’s home, the sushi restaurant where Scaramella and Litvinenko met, and a hotel where Litvinenko held meetings on Nov. 1. The British Health Protection Agency has asked anyone who ate at the restaurant or the hotel to contact health officials.

“The Health Protection Agency continues to reassure members of the general public that the risk of having been exposed to this substance remains low,” the health agency said in a statement.

Litvinenko was an outspoken critic of Russian President Vladimir Putin, and he issued a deathbed statement accusing Putin of orchestrating his death. The Russian government has denied any involvement.

British news agencies reported Friday that three pathologists were conducting an autopsy on Litvinenko, wearing suits to protect against any radiation risk.