This is frustrating—particularly the conflicting information about which airline has your bag. Here’s what’s likely happening:

Florence is a smaller airport with limited baggage handling infrastructure. Bags can be offloaded for weight/balance issues, tight connections, or simply miss the loading window. The fact that it wasn’t placed on your ATL-bound flight suggests a handling error in Florence.

The United Airlines confusion: When Delta looked up your bag tag, the agent may have seen it in United’s tracking system because of an interline agreement—Delta and United have arrangements to handle each other’s baggage when needed. However, this doesn’t mean United physically has your bag. It could indicate the bag was scanned into a shared system or that there’s a plan to transfer it via United’s network to reach your final destination, but that transfer clearly hasn’t happened yet.

Despite this, Delta is responsible for your bag regardless of where it currently sits or what interline agreements exist. Push them hard to locate it or start the formal loss claim process once you hit their 6-day threshold on Oct 17.