If the owner of the computer has been arrested and is the subject of a criminal investigation, he can not be forced to give the police a password as it is a violation of his 5th amendment right.
A typical Miranda warning includes the following: "You have the right to remain silent (when being questioned)."
Dislaimer: I am not an attorney. I am not your attorney. This advice is not intended to apply to your specific situation. DO NOT FOLLOW IT.