The educational requirements to become a police officer in the United States depend on which police force you want to join. However, in order to join the federal law enforcement agencies, you must have a college degree. To join the state or city or county law enforcement, you must have a high school diploma or a GED. To join the state law enforcement, you need to have 60 college credits, and for city or county you must possess 45 college credits. These are the general academic requirements to become a police officer. However, the requirements may differ depending on the law enforcement agency that you want to join.
A four year college degree in any field of study will qualify you for a job in most federal, state and local law enforcement agencies. However, if you don't have a college degree, most federal agencies will accept three years of responsible work experience. For example, the secret services would require that you have criminal investigation experience whereas on the other hand the U.S. marshal service will accept you if you have three years of experience in a supervisory position. Serving three years as a law enforcement officer will not necessarily qualify you to join particular federal agencies.
The educational requirements will really depend on which agency you are interested in joining. Like the educational requirements for joining the criminal investigation agency will differ from that of becoming a U.S. marshal. If you want to join a particular agency, then you need to check out what are their educational requirements. Most federal agencies require that you go through the educational training required for each field among the federal agencies.
In some agencies, for example in the FBI there are various departments and each department specializes in handling different types of crime. This agency deals with terrorism, foreign counter intelligence, cyber crime, organized crime, white-collar crime, and a number of other federal offenses. Therefore if you want to join this agency you need to be qualified in an area of specialization. To join cyber crime would require having educational qualification in information technology, and to join white-collar crime would require qualification is business administration.
Once you join the police force you can request for training in a particular field of interest. However, it will be up to your superiors to recommend you for that training or not. Of course, the agency that you want to join will also evaluate whether or not you are fit to be trained by them.