Generally speaking, it's a police officer employed by the state. For instance, you may have city officer, county officers, and state troopers. In my state, state troppers are charged with "enforcing federal and state laws relating to traffic, investigating accidents, and criminal interdiction on roads and highways."
Yes,a state police can investigate a state case.
Maryland State Police was created in 1921.
to be a police officer you need to attend a state approved police academy and be state certified in the state you live in.
In a police state, the government would control the actions of police.
It was the Pennsylvania State Police.
Virginia State Police was created in 1932.
Indiana State Police was created in 1933.
Louisiana State Police was created in 1922.
Idaho State Police was created in 1919.
California State Police was created in 1887.
Massachusetts State Police was created in 1865.
Oregon State Police was created in 1931.