The Federal Government;

You are probably looking for a Section 1983 claim.

federal law suits against police for violations/us code

42 U.S.C Section 1983 is the law. It was enacted in the Civil Rights Act of 1871. When you sue an official under Section 1983, you are usually suing for violation of a constitutional right, not a statutory one, so there aren't a lot of laws to consider but there are a lot of court cases. There are a lot of exceptions to 1983, including absolute immunity for legislators, court witnesses, etc, and qualified immunity for executive officials. All of these come from cases, not laws.

The liability was under federal law to answer your exact question.

You are probably looking for a Section 1983 claim.