Which agency investigates the most serious violations of the Privacy Rule?

The agency responsible for investigating the most serious violations of the Privacy Rule under HIPAA is the Department of Justice. This is because the Department of Justice handles criminal enforcement and prosecution of HIPAA violations that lead to significant harm, such as cases involving intentional misuse of protected health information (PHI) or actions that compromise the privacy rights of individuals significantly.

In the context of HIPAA, while other agencies may have roles related to health and welfare, they do not have the same level of authority regarding the criminal prosecution of privacy violations. The Department of Health and Human Services may oversee compliance and impose civil penalties for violations, but the most severe breaches that might involve criminal penalties fall under the purview of the Department of Justice. Hence, the focus on criminal investigations and enforcement makes this agency the correct choice for handling serious violations of the Privacy Rule related to HIPAA.