Mail At My College Keeps Arriving Opened/missing Money

Location: California, USA

Ever since I began living at the dorms over a year ago, any handwritten mail for many tenants has been arriving with a corner torn open and several students have reported their cards that were meant to come with gift cards or cash are missing the money. This has happened to me personally twice. A letter from my grandfather never arrived and a letter from my grandma arrived with a corner torn and the money missing. Today, it happened to my best friend who received a handwritten letter from a nonprofit. There was no money inside, but it seemed they checked because it looked like a letter from family. Since living here I have become an RA. I sort the mail myself sometimes and i see mail from family or mail that looks like a paycheck arriving with the corners torn. I am fairly confident it is not anyone at the dorms doing this as it is a small college, but someone along the mail system. This mail is arriving from very different locations (my grandparents live in different states) so it must be at a local level.

Does anyone know what i can possibly do to start tracking the source? What action am I allowed to take? I have no photos of the incidents before today but i plan to start gathering evidence and asking friends if any notice this happening to send me evidence. Also, could i get in trouble by trying to catch them myself? I am thinking like using counterfeit bills, something that will stick to their hands (glitter etc) or something else inside the card.

Any advice is appreciated.