The rift between New York City Mayor Bill de Blasio and the NYPD took to the skies Friday. A small plane flew around the city bearing a banner that read “De Blasio, our backs have turned to you.”

The sign referred to a silent act of defiance exhibited by some police officers who turned away from the mayor when he appeared at a press conference at Woodhull Hospital last weekend. De Blasio was speaking about the killings of officers Rafael Ramos and Wenjian Liu by Ismaaiyl Brinsley, a tragedy that police unions have blamed on the mayor and his support for New Yorkers who have been protesting the killing of Eric Garner and the non-indictment of the officer who choked him to death.

Blogger and former cop John Cardillo ran a statement on his website from the people who paid for the banner and the plane, which he says is an anonymous group of current and retired members of the NYPD. “We are outraged by the mayor’s incendiary rhetoric, and for facilitating the current hostile climate towards the NYPD,” the statement reads. “We understand that the department and even our own unions can only go so far in their public condemnation of the mayor as to not irreparably damage the working relationship with the city, or future contract negotiations.”

Police reform was a major element of de Blasio’s mayoral campaign, and he has already begun efforts to end the NYPD’s stop-and-frisk program. The department is also undergoing a massive retraining effort started by the mayor. But police unions have painted the mayor as being anti-cop since the campaign, and the national focus on police brutality has brought increased attention and emotion to both the protests and reform efforts.

In a statement released by the mayor’s office Friday afternoon, Deputy Press Secretary Wiley Norvell said, “This is a time to think about the families and honor our fallen officers. Dividing people won’t help our city heal.”