i'm in massachusetts.
this same type of firing has happened before at my place of employment where employee is told of a serious complaint, there is the talk, then the written warning, the monitoring, and so on, all based on the few supposed Verbal complaints. eventually the employee is so harassed they take a severance, sign away saying they'll never sue and never discuss...
Hearsay is a legal term used in courts to determine whether or not testimony can be admitted. Outside of court, "hearsay" has minimal legal implications.
Your employer can investigate a complaint and rely on their opinion of it when deciding whether or not to take disciplinary action against the employee. If you are an at will employee, the employer can fire you for any reason (except one which is discrimination). The employer is not required to give the employee a warning or performance improvement plan (unless the employee handbook states otherwise).
This is the most specific answer I can give based on what you've stated in your question. You may have additional rights depending on the facts. I suggest you work with the company to be the type of employee they want. Or if there are more facts than you've stated, please get personal legal advice.
Lawsuit / Dispute Attorney
As an at-will employee, you can quit at any time for any reason, you can be fired for any reason, a rotten reason, or no reason, except unlawful discrimination. If you do not have an employment contract or union to represent you, your recourse is limited. Discrimination based on age, gender, race, religious beliefs etc.... may give rise to a different answer.
You might find my Legal Guide helpful "Workplace Discrimination: A Basis for Wrongful Termination Claims"
You might find my Legal Guide helpful "How to Choose a Lawyer for you.”
You might find my Legal Guide helpful "What Do I Tell My Lawyer"?
No one can know what the record is in the case because online we cannot find out any details. Check with a lawyer in your locale to discuss more of the details.
If you have a discrimination theory about retaliatory discharge or hostile workplace conditions you'll need a lawyer. Religious belief, age, gender, race etc. all are discriminatory reasons for which you cannot be legally fired and upon which you might base a wrongful termination suit.
Good luck to you.
God bless. Best of luck to you.
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