Mystery Lingers in Russian Poisoning Case

March 28, 2007 — -- Marina Kovalevsky and her daughter, Yana, were on a picture-perfect trip to their native Russia last month when the trip took a shocking turn.

"The trip was fun," said 26-year-old Yana. "We were spending time with family and friends we don't typically get to see."

Then they both became suddenly and mysteriously ill.

"I got severe pain, stomach cramps," Yana said. "We were both getting a lot worse, like, really quickly."

Theirs is the latest in a string of mysterious Russian poisoning cases, but how did a family from Los Angeles get caught up in the international intrigue?

The Kovalevskys were told they'd been poisoned with an odorless, colorless and very toxic substance called thallium.

"Thallium is one of the more toxic metals. If you run the concentration up high enough, it can kill people," said Dr. Joseph Landolph of the University of Southern California.

Thallium has a dark history. Saddam Hussein was known to use it against his enemies, and it was a poison of choice for Soviet-era KGB assassins.

In November, thallium was suspected and then ruled out in the poisoning death of former Russian spy Alexander Litvinenko. Litvinenko swore the Russian government was behind his poisoning -- an accusation that it denied.

The Kovalevskys feared the same fate.

Marina's brother, a doctor like her, rushed an antidote to Russia -- a swift move that quite possibly saved their lives.

"Someone else maybe was a target and they were at the wrong place, wrong time," said Dr. Leon Peck, Marina's brother.

While Marina and Yana's health improved, the mystery lived on.

The mother and daughter returned to Los Angeles still not knowing how they had come into contact with the poison. More troubling, they still don't know why.

"Even now, I'm not sure how it happened, but definitely we got this poison in Russia," Marina said.

An attorney for the Kovalevskys said, "Whether this is accidental or intentional can't be ruled out at this time."

The FBI confirmed to ABC News that it was still investigating the case.