I wanted to know if us citizenship was required in order to become a police officer

From searching the web they recruit Citizens or some states will recruit LPR's if the LPR is eligible to apply for citizenship, or in the process of becoming a citizen.

http://www.google.com/search?q=police+officer+us+citizenship

Look at the chart here: http://www.copcareer.com/basicrequirements.htm Some states don't have a check in the citizenship column, but they may be like California, they recruit LPRs that can become a citizen by the time they become an officer.