corroborate

This term carries a strong connotation of legal or scientific validity, implying that a claim is strengthened by the addition of external, independent evidence. It is far more formal than "confirm" or "back up" and is typically used in contexts where a high standard of proof is required, such as in courtrooms, police investigations, or academic research. While "verify" focuses on checking the truth of a single piece of information, this word emphasizes the process of adding supporting layers to a narrative or theory to make it more believable or indisputable.