Which agency investigates criminal violations related to explosives and firearms?

The correct answer is the agency that specializes in overseeing and enforcing laws related to the regulation of explosives and firearms, which is the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF). This agency is tasked with investigating illegal trafficking in firearms and explosives, along with enforcing federal laws concerning the manufacture, importation, distribution, and possession of these materials. The ATF also plays a crucial role in preventing violent crime associated with the misuse of firearms and explosives.

In contrast, the Drug Enforcement Administration (DEA) focuses primarily on enforcing the controlled substances laws and regulations, which does not include the regulation of firearms or explosives. The Federal Bureau of Investigation (FBI) has a broader scope that includes counterterrorism, cybercrime, and other federal law enforcement responsibilities but does not specialize in explosives and firearms. The U.S. Marshals Service is primarily responsible for apprehending fugitives and managing federal prisoners, and while they have a significant role in law enforcement, they do not have specialized jurisdiction over explosives and firearms violations.

Thus, the ATF is the agency designated specifically for matters involving explosives and firearms, making it the correct choice in this context.