TL;DR: "while a new jail in Nashville was still under construction, staff discovered missing keys and other anomalies. Surveillance footage eventually revealed that someone had repeatedly disguised themselves as a construction worker and entered the building many times. Inside, they hid weapons, tools, and escape items in walls and rooms around the facility."

Reminds me of the US Embassy in Russia that was built, by Russians, who embedded thousands of spying devices within the building itself. It took 27 years to build, then debug the building.

And it was the person both least, and most suspected.