Access to police files...
Police tend to hold their files pretty closely and do not often let members of the general public see them without good reason. Family members of a victim might be able to see the files, depending on circumstances but often family members are among the first to be investigated, and police might not want to give up too much information.
The Freedom of Information Act was passed about 1974, but it only applies to Federal Records and information held by Federal agencies and offices. So, FBI files are subject to it, but not local or State Police files.
It also depends a lot on whether a file is considered still open, or closed.