Here the answer you've been waiting for

It is: Ms.Blair Warner, in the Chugalug Bar, with the socket wrench.
HERE'S THE STORY

When Andy tried to hide from the murderer in the Chugalug the ID guy let him pass because he bribed him. Blair snuck in through the back, but before she did, she saw Jo's tool box and picked up the socket wrench, found Andy inside snuck up behind him and killed with a blow to the head.

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Tootie:Will Danny open the door?

Mrs.Garrett: No, that kid would've been great at the Alamo!