Employers should beware of being too quick to believe an employee who accuses a co-worker of wrongdoing. If the accuser has an illegal motive (such as discrimination or retaliation), and if the employer is “negligent” in investigating before taking action against the co-worker, then the employer could be legally responsible.

So says the U.S. Court of Appeals for the Second Circuit, and at least one other federal appeals court.

Here’s what allegedly happened in Vasquez v. Empress Ambulance Service: Tyrell had the hots for his co-worker Andrea, and made some inappropriate comments to her, which culminated in his sending her a very inappropriate text message. Arguably worse than Anthony Weiner’s latest, although, to his credit, Tyrell didn’t do it while his kid was sleeping in bed beside him. Andrea became upset and complained to her supervisor. The supervisor asked her to put her complaint in writing, and Andrea obliged, but while she was doing it, Tyrell walked by and figured out what was going on. First he went to a male co-worker and asked the co-worker to lie for him. The co-worker refused. Then Tyrell printed out a bunch of fake text messages indicating that Andrea was coming on to him.

What’s “cat’s paw”? That’s when an employee with an illegal motive manipulates an innocent employer into taking action against another employee. The expression comes from an Aesop’s fable in which a monkey tricks a cat into reaching into a fire for chestnuts. After the cat pulls the chestnuts out of the fire, the monkey eats them all, and the cat is stuck with a burned paw and no nuts.

The U.S. Supreme Court recognized the “cat’s paw” theory of liability for discrimination in Staub v. Proctor Hospital, a 2011 case involving supervisors who were upset about the plaintiff’s military service. The supervisors accused the plaintiff of violating the hospital’s corrective action policy, and the Vice President of Human Resources fired the plaintiff based only on the alleged policy violation. But because the supervisors had illegal motives, the hospital was liable for discrimination under the Uniformed Services Employment and Reemployment Rights Act under a “cat’s paw” theory.

Management confronted Andrea with the fake texts and did not listen to her explanation or give her an opportunity to prove that the texts were fake. Instead, they fired Andrea for sexually harassing Tyrell.

Andrea sued her employer and Tyrell. A federal judge in New York threw out the claims against the employer, saying the employer couldn’t be liable because, even though Tyrell may have had an illegal motive, he wasn’t in a supervisory position.

But a three-judge panel of the Second Circuit disagreed. According to the appeals court, an employer is not normally responsible for the wrongdoing of non-management employees. However, the employer may be liable if it failed to investigate, or conducted a negligent investigation.

At this very early stage of the lawsuit, the court had to assume that all of the allegations in Andrea’s lawsuit were true. Based on that assumption, the court found that she had sufficiently alleged negligence on the part of the employer, so the lawsuit will go forward. As always, it’s possible that the employer will be able to get the case dismissed later, or will win at trial.

So, employers – never skimp on your investigation unless there is no dispute about what happened.

A word about text messages: Text messages are strong (and often damning) evidence, but they can be altered or fabricated. My own philosophy about this is that once an employee presents text messages supporting his claim, the burden is on the accused employee to claim — and prove — that that the texts are not genuine. But the accused employee should have a chance to present that proof, and the employer ought to consider it before making a final decision.