I had a similar intrusion into my luggage when flew last time BUF-JFK-SAN. Amazingly I didn't lost anything because these guys even didn't close the bag properly and made a lot of mess inside.
I know it was TSA, they attached the small sticker on outside surface of the bag. The idea with stickers is absolutely stupid, my bag was slightly wet and the seal come unstuck when I just touched it.
Damaged Baggage
Written notification will be placed inside your luggage or a seal may be placed on the outside of your luggage, if the Transportation Security Administration (TSA) made a random search of your bag.
Good luck with your claim, mine looked like hopeless. Nevertheless, I wrote a general claim on delta.com and got some miles from DL for headache.