If you know that a crime is about to be committed, what should you do?
Report it to police?
Wait until it occurs and then report it?
If you report it ahead of time, what should the police do?
Make plans to catch the criminals in the act?
Contact one of the people involved and tell him (or her) that the police know what's going to happen?
Send an officer to stand guard at the location so that the crime doesn't occur?