The goal of this sort of inflammatory commentary is frequently to cause the employee under investigation to react in an overtly hostile manner, whereby the employer can use the employee's reaction as proof that the employee is a danger to others, even if he/she never committed the complained about act.
So, be careful about overreacting to the claims. It's usually better to laugh about these types of absurdities and treat them as frivolous/silly (e.g., "They said what? That's pretty funny. Listen, if you have nothing other than this nonsense to discuss, I really need to get back to work now!" -- and, then you get up and walk out of the meeting, before they can stop you -- which makes everyone but you look like a moron). It may be too late for you to do this, but it may be worth considering in the future.
Back to your original question, you can't sue the employer for defamation, because even though, legally, a republication of another person's defamatory statement is actionable, there are exceptions for employers in carrying out routine management duties. You would never get this sort of case to trial.
Now, if you can identify the complainant, you could sue them personally -- but, they may not be able to pay a defamation judgment, so you could spend a lot of time and money suing and then never collect anything. So, you need to carefully consider your target defendant to determine whether or not they have sufficiently "deep pockets" so as to be able to pay a judgment. This may not be easy to do, unless you know the coworker well enough to have a good idea about his/her financial circumstances.
Hope this helps.
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