Where did Kevin Garnett's doping accusations against LeBron James come from?

NBA legend Kevin Garnett accused LeBron James of using performance-enhancing drugs (PEDs) during a podcast with Paul Pierce. Garnett hinted at LeBron's connection to the Bay Area Laboratory Co-operative, implying he used "new juice" from the group. This echoed earlier claims by MMA fighter Chael Sonnen about erythropoietin (EPO) use.