An Overlapping Network
Since there are variations in every mission statement that exists for each of the executive branch departments, there are a variety of ways laws are enforced at the federal level. Each agency is answerable through their chain of command to the secretary of the department, who drafts that department's mission statement. While it might seem as though this lack of uniformity would lead to chaos and confusion, it is actually beneficial in maintaining an overlapping network of activities that help thwart criminal activity. Although procedural law enforcement is practiced the same across the board, the variations in missions for the agencies and bureaus helps to keep crime off balance. There are gaps, but they are not wide or frequent.
What one agency is not tasked to handle, another agency is quick to pick up. When gaps in the network are discovered, they are quickly closed by a shifting of executive responsibilities, with a corresponding bump in funding to accommodate the changes.