The FBI made an early morning visit to the headquarters of the NYPD sergeants' union.

Multiple sources tell NY1’s Dean Meminger this is part of an ongoing investigation into the Sergeants Benevolent Association and its controversial president Ed Mullins.

The FBI would only say there is an investigation. A spokesperson for the U.S. attorney in Manhattan would only say that a "law enforcement action" was being conducted at the location.

One source with knowledge of the probe tells NY1 that the FBI and federal prosecutors in Manhattan are looking into possible mismanagement of funds.

Mullins, who is fiery and often takes extreme positions to defend his members, is also currently in trouble with the police department.

He is facing a departmental disciplinary trial for releasing information on social media about Mayor Bill de Blasio’s daughter’s arrest during a Black Lives Matter protest.

He has also used foul language to describe the former city health commissioner, Dr. Oxiris Barbot, and Bronx Rep. Ritchie Torres.

His police departmental trial will resume later this month.

In his daily briefing, de Blasio confirmed that the FBI raided the SBA headquarters and that it was connected to an ongoing investigation, but did not have any additional details.

He did criticize Mullins, calling him "divisive" and "destructive."

"A lot of what he has done is really, really destructive," de Blasio said. "Especially in the middle of a crisis where we're trying to unify, we're trying to get people through together, he's been a divisive voice."

Regarding the FBI investigation, this is the first time agents showed up to the union’s headquarters for this probe.