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,” Cossairt 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.