S.H.I.E.L.D., formally known as the Homeland Strategic Defense, Attack and Logistics Support Agency, was an international organization directly under the United Nations Security Council that specialized in handling supernatural incidents. It held dual law enforcement authority within and beyond the United States, with an extremely broad jurisdiction, but there was one strict requirement for cases under its authority: they had to be supernatural incidents.
Determining whether an incident was supernatural required cooperation from multiple parties. When a case emerged in a certain area, the investigative order for law enforcement agencies within a state began with local community police, then higher-level police departments, then the state Bureau of Investigation. If state law enforcement agencies could not determine the cause, they would request further assistance, and the case would end up in the hands of the Federal Bureau of Investigation.
The Federal Bureau of Investigation would gather information more broadly, communicate with local officers, and even participate directly in the investigation. If they then believed the case exceeded the scope of ordinary human crime, it would be submitted to the department responsible for supernatural incidents.
However, S.H.I.E.L.D. was not the only department that managed supernatural incidents. Nearly every state had a Special Security Administration, abbreviated as SSA, to handle the many ambiguous supernatural cases within the state.
The reason such departments existed was entirely because many supernatural cases were difficult to define. If a particularly clever serial killer used religious elements in the staging of his murders, then before he was caught, it was easy for people to mistake it for a supernatural incident. Such cases needed to be handled by each state's Special Security Administration.
After all, Marvel was a universe with superheroes. There were also quite a few capable people among the public who could not be called superheroes but