you will need to find where the varments are getting in at and close it up. they can find very small holes in eaves, roof vents, etc. you might try using the peanut butter type rodentcides that make them thirsty so they will leave the attic to find water before dying. otherwise, you will have to follow your nose to the carcus. good luk
I would avoid using poisons as dead mice or squirrels are worse than live ones. If it is mice you can use glue boards, snap traps and so on that can be removed. For squirrels, the task is much more difficult.