Illegal activities by detectives?

Is it legal or illegal for a detective or police officer to open a mailbox without the permission of the owner of the residence that the mailbox belongs to. or must they have a court order or warrant. There was an investigation done that the police admitted to having put one of their car keys into the mailbox key slot. This opened the mailbox and they took out mail, noted who the mail was to and where it was from. They put the mail back, went to the address that owned the mailbox, spoke to the people, of course not mentioning that they had just opened the mailbox. After the detectives finished speaking with the people at the address in question, they returned to the mailbox, only approx 10 minutes later, to find that letters that they had recently observed and noted were gone. I have all of this information in an affidavit. Please assist me with this information as I believe this was an illegal act that the detectives exercised.