Not only did they lose their son Andrei in the auditorium, but they also lost their health.
"Now, even the smallest movement takes a huge effort," says Nikolai, who's been battling kidney cancer, liver and stomach ailments, and neurological problems.
They were members of the audience at Moscow's Dubrovka Theater, which was taken hostage by Chechen rebels on October 23, 2002.
To end the standoff, special forces pumped a secret narcotic gas into the building. Although it incapacitated the hostage takers, it also affected the entire audience. As a result of the gas and a bungled first-aid effort, 130 of the hostages died. Those who did survive, inhaled large quantities of the knockout gas, the ingredients of which remain secret.
Russian president Vladimir Putin maintains the gas was harmless, yet no one has ever systematically studied the health of the survivors to find out if that's true.
But Knobel's team found about 100 of the 800 people who lived through the gas attack here, and asked them questions about their health before the siege, afterwards, and now. Almost all of them reported having significant medical problems since the attack — problems they blame on the gas.
Tamara Yusipova was a cleaner at the theater. Now, she's on disability.
"I have heart, liver and stomach problems. I am losing my hearing and vision, and my kidneys failed," she says. "Officially the doctors don't say this all is the result the gas, but I think they understand it is."
Some physicians who treated the survivors think the gas has long-term consequences — but they're afraid to speak out because Moscow medical authorities ordered city doctors to play down the effect of the gas.
A doctor who treated several of the patients — and who would only speak on the condition on anonymity — had written at the time of the incident that one patient had been poisoned by the toxicity of the gas. But her boss told her to remove the diagnosis of poisoning, or lose her job.
"I was told, demanded, ordered, to cross out from final report the word toxicity. I was ordered to put the only diagnosis: 'Victim of crime and terrorism,' she said.
Both federal and Moscow city medical officials turned down CBS' requests for an interview.
The Russian government argues the storming of the Dubrovka Theater was essentially a success, because more than 80 percent of the hostages were saved. Without the knockout agent, they say, everyone inside might have died.
With the fourth anniversary of the hostage taking coming up next week, the survivors are calling for a thorough study to determine if the chemicals used on them were in fact poisonous.
After all, the recipe for the gas remains a Russian government secret — and it may be used again.