Accused would certainly be appropriate there, as would defendant, but prisoner isn't necessarily wrong, because it doesn't imply that the person has been convicted.
Many people are held in prison before and during their trials, so they can be referred to as prisoners, even though the legal proceedings haven't determined their guilt.
It's possibly more accurate to call these people detainees than prisoners, but they're commonly referred to as prisoners.
Edit: all this is from a U.S. perspective; it may be different in other English speaking countries.