re: Need advice on handling workplace violence/threat/harassment.

What an absolute shite company. Regardless of the outcome, you should not continue to work there.

Market is pretty hot right now, follow through with the investigation, but plan your exit.

Reach out to HR. Go up another level. The Plant Manager is not above HR Leadership or lawsuit. There’s a reason most companies separate HR from Production. The Plant Manager will not risk his job for this BS Manager causing trouble. But either way, this doesn’t end well for you. It’s the right thing to do, and toxic work culture has allowed it to happen and you to become the enemy because of reporting it.