MOSCOW – Police in Moscow are trying to determine how and why an American mother and daughter visiting the Russian capital were poisoned with thallium, a dangerous substance that can kill in amounts as small as a gram.

Russian authorities said tests showed that Marina Kovalevsky, 49, and her daughter, Yana Kovalevsky, 26, were exposed to thallium, though they are still investigating when and where the poisoning occurred.

Both women had been hospitalized at Moscow’s Sklifosovsky Clinic since Feb. 24. The Russian news agency Interfax reported that the two women were well enough to leave the hospital Wednesday and had boarded a flight bound for the U.S.

Asked about the case, a U.S. Embassy spokesman read a statement that said the women “were hospitalized in Moscow for possible thallium poisoning and were hoping to return to the U.S. soon.”

Marina Kovalevsky is a doctor with a practice in suburban Los Angeles. She and her daughter emigrated from the Soviet Union in 1989 and now live in Studio City, Calif. The Russian newspaper Kommersant reported that the two women were vacationing in Moscow last month and began feeling ill after a party Feb. 23. Both women complained of strong pain and numbness in their legs, Kommersant reported.

Investigators with Russia’s Federal Security Service interviewed the women and are looking into whether the poisoning was linked to the theft of some jewelry they owned, according to Kommersant.

Thallium is a highly toxic, malleable metal used in the making of pesticides, rat killer and semiconductor components. Tasteless and odorless, it is used as a poison in powder form. It attacks the nervous system and causes damage to the stomach and kidneys.

Saddam Hussein’s intelligence agents reportedly used thallium as a poison against the regime’s enemies, and during the Soviet era the KGB also relied on the substance as a poisoning agent.

When former Russian spy and Kremlin critic Alexander Litvinenko became ill last fall, doctors first believed he had been poisoned with thallium. Later tests showed he had been exposed to polonium-210, a rare radioactive substance that can prove fatal in minute amounts.

Before his death Nov. 23, Litvinenko accused Russian President Vladimir Putin of being behind his poisoning, a charge the Kremlin has vehemently denied.