My roommate is stealing from me, what can I do?

I have three roommates and we all don't get along with one. I came home one day and noticed my things were in places I didn't leave them (I leave my bedroom locked), and then I noticed that my door looked like someone was shoving a card through it to get it open. I now realize that I'm missing a pair of pants. I know who would have done it, but I can't prove it. Is this worthy enough to go to the police? I talked to the landlord and they reinforced my lock but still won't kick her out.