Unfortunately, you are right.
Howell and his followers thought they were under attack. At one point, he even called 911.
A friend of mine followed the whole thing very closely. He believed that the whole thing went south when BATF agents executed dogs that were either attacking them or believed to be dangerous.
Those initial gunshots apparently set off the fire-fight. It isn't clear who over-reacted to the shots first, but I think my friend is right about what triggered it.
IIRC, the initial gunshots were when the BATF van came skidding up to the front door. Koresh was coming out the door, unarmed, holding a baby. The BATF opened fire with full-auto rounds sweeping across the doorway, killing the baby and wounding Koresh with a couple or three rounds.
The door and its surrounding frame, with the incriminating bullet holes were taken away and destroyed at the beginning of the clean-up project.