USADA had given Armstrong until Wednesday to decide whether he would testify under oath as part of the investigation, CNN said.
Armstrong's attorney, Tim Herman, said in a written statement Wednesday that Armstrong won't cooperate.
"Lance will not participate in USADA's efforts to selectively conduct American prosecutions that only demonize selected individuals while failing to address the 95 percent of the sport over which USADA has no jurisdiction," Herman said the statement.
USADA Chief Executive Officer Travis Tygart said Wednesday that Armstrong is concerned about criminal and civil liability if he assisted the USADA.
Armstrong was stripped of his seven Tour de France titles after the USADA accused him of doping. After years of denial, Armstrong admitted to using the drugs in a January interview with Oprah Winfrey.