As Russian soldiers surrounded Bucha in early March, Yershova hid in an apartment with two other friends.
Her mother left a message on Facebook begging anyone who knew what had happened to her to get in touch.
Dereko also believes Yershova was sexually abused by Russian troops.
The Dereko family's agonizing wait for answers reflects the rising anxiety amid reports of wartime rape in the country.
Human rights groups and Ukrainian psychologists who CNN spoke to say they have been working around the clock to deal with a growing number of sexual abuse cases allegedly involving Russian soldiers.
"Rape is an instrument of war against the civilian population -- an instrument of destruction of the Ukrainian nation," she said.
Human Rights Watch reported at the time that Russian soldiers had raped Chechen women in Russian-held areas. »