The primary objective of the placebo-controlled period is to evaluate the safety and tolerability of BIIB092 in participants with mild cognitive impairment (MCI) due to Alzheimer's disease (AD) or with mild AD. The secondary objectives of the placebo-controlled period are to evaluate the efficacy of multiple doses of BIIB092 in slowing cognitive and functional impairment in participants with MCI due to AD or with mild AD, and to evaluate the immunogenicity of BIIB092 after multiple doses in participants with MCI due to AD or with mild AD. The primary objective of the long-term extension period is to evaluate the long-term safety and tolerability of BIIB092 in participants with MCI due to AD or with mild AD.