Russia's baseless claims about secret American biological warfare labs in Ukraine are taking root in the U.S. too, uniting COVID-19 conspiracy theorists, QAnon adherents and some supporters of ex-President Donald Trump.

Despite rebuttals from independent scientists, Ukrainian leaders and officials at the White House and Pentagon, the online popularity of the claims suggests some Americans are willing to trust Kremlin propaganda over the U.S. media and government.

Like any effective conspiracy theory, the Russian claim relies on some truths: Ukraine does maintain a network of biological labs de...