Open Source Intelligence community.
GUESSES:
Video surveillance--she put them in a mailbox, and they tracked her with other video recordings until they could see her face, or maybe time her entry into a building with keycard access, etc.
Someone ratted her out.
She took them in a bundle to the PO counter, rather than use a mailbox.
Fingerprints on the paper.
I actually found out the answer. They got a search warrant to find the email associated with the orgs Instagram account and IP addresses. They tracked the IP addresses to her home and phone number (not sure how they did that). Then somehow they found on her phone a note that had the addresses of the board members. I'm assuming they got access to the phone in person ?
Bet a judge tosses this.
She/her organization bragged about having sent those cards on Instagram, cops got their IPs from Meta which led them to her phone where they found more incriminating stuff:
A search warrant requested data from Meta that revealed IP addresses associated with the IUDC Instagram account. A search of McComish’s phone, which was linked to the account, revealed searches for trustee addresses and a note that nearly matched the language on the holiday cards, according to the affidavit.
https://www.idsnews.com/article/2025/05/student-charged-intimidation-for-letters-iu-administrators