BOSTON (AP) — A watchdog agency is investigating whether Massachusetts’ top federal prosecutor violated a law that limits political activity by government workers for attending a political fundraiser that featured First Lady Jill Biden, according to an email reviewed by The Associated Press.

The U.S. Office of Special Counsel said in the email last week that it has opened an investigation into a potential violation of the law known as the Hatch Act after Republican Sen. Tom Cotton raised concerns over U.S. Attorney Rachael Rollins’ attendance at a July Democratic National Committee fundraiser in Andover.

A spokesperson for Rollins declined to comment on Wednesday. The Massachusetts U.S. attorney wrote on Twitter after the Boston Herald reported that she attended the July fundraiser in Andover that she “had approval to meet Dr. Biden” and left early to speak at two community events.

The investigation was first reported by Reuters.

Cotton, a fierce critic of Rollins who had sought to block her confirmation, wrote a letter to the inspector general of the Department of Justice in July asking for an investigation into whether the U.S. attorney’s actions violated the law.

The Hatch Act aims to curb the influence of partisan politics in the operation of government agencies regardless of which party is in office, although an Office of Special Counsel report last year said that Trump administration officials repeatedly violated the law without consequence.

In the case of Senate-confirmed presidential appointees — like U.S. attorneys — the watchdog submits a report to the president and it’s up to the president to discipline them if there was a violation.

On Tuesday, Attorney General Merrick Garland said in a memo to employees that the Justice Department will no longer allow political appointees to go to fundraisers and other campaign events.

Previous longstanding policy had allowed them to attend political events “in their personal capacities if they participated passively and obtained prior approval,” the memo said.

Garland said the restrictions are to ensure that politics “both in fact and appearance” won’t affect the way the law is enforced or how inquiries are carried out.

Rollins, the first Black woman to serve as U.S. attorney in Massachusetts, previously served as the top prosecutor for Suffolk County, which includes Boston.