Re: UK Citizen looking for Forensic Investigator Job in the states
To work as a non-permanent resident in the United States you would have to find an employer who will sponsor you for a temporary worker visa for a job opening they have. Since there are a lot of USA citizens applying for the CSI and forensic jobs, it will be difficult for you to get an agency to go to the trouble of sponsoring you unless you have a lot of experience or a speciality they really want. And most agencies want career employees, not temporary workers.
Another way to work legally in the United States would be to get a Green Card (Permanent Residence). It is possible to get a Green Card through a job offer. (You may be eligible to become a permanent resident based on an offer of permanent employment in the United States. Most categories require an employer to get a labor certification and then file a Form I-140, Immigrant Petition for Alien Worker, for you.)