Three Vermont state troopers have resigned following allegations related to fake COVID-19 vaccination cards.

Per a statement released by the Vermont State Police, two troopers—identified as Shawn Sommers and Raymond Witkowski—handed in their resignations on Aug. 10. The previous day, another trooper had “raised concerns with supervisors” about their conduct. Trooper David Pfindel, meanwhile, resigned effective Sept. 3 after additional investigation by the Department of Public Safety.

In a statement, Col. Matthew T. Birmingham—director of the Vermont State Police—said he was “upset and disappointed” by the alleged behavior.

“If these allegations are proved to be true, it is reprehensible that state troopers would manipulate vaccination cards in the midst of a pandemic, when being vaccinated is one of the most important steps anyone can take to keep their community safe from COVID-19,” Birmingham said, adding that he’s “embarrassed” by this situation that has “tarnished” the department’s reputation.

The now-former troopers are accused of having “varying roles” in the creation of the fake vaccination cards. Local authorities have since referred the incident to the FBI, as well as the U.S. Attorney’s Office in Burlington. Tuesday, Vermont State Police confirmed that an FBI investigation is currently in progress.

“Based on an initial internal review, we do not believe there is anything more the state police could have done to prevent this from occurring,” Vermont Public Safety Commissioner Michael Schirling said. “As soon as other troopers became aware of this situation, they raised the allegations internally, and commanders took swift and decisive action to hold these individuals accountable and report this matter to federal authorities.”