For the second time this week, a thin blue line flag was spotted on NYPD property. The framed banner was seen hanging from the wall of a transit police station at 148th Street and St. Nicholas Avenue in Harlem on Sunday afternoon. That sighting comes days after the NYPD said they were investigating the flag's appearance at an NYPD Highway station house in Brooklyn near the Belt Parkway.

The flags, featuring a horizontal blue line surrounded by black, are closely linked to Blue Lives Matter, a countermovement formed in response to Black Lives Matter.

Displays of the symbol have sparked controversy in other cities, with proponents arguing that the flag represents a tribute to law enforcement, while police reform groups claim that the flag denotes racism and a culture of misconduct. In recent years, the flag has appeared frequently at neo-Nazi and white supremacist rallies, including the deadly Unite the Right rally in Charlottesville.

It's unclear when the two flags were to the police facilities. Last week, NYPD spokesperson Al Baker told Gothamist that “the commander of our NYPD Highway district is looking into the matter" of the flag in Brooklyn. The NYPD has not yet answered our questions about the flags in Harlem, and they have not provided an update on the investigation of the first flag. A person who answered the phone at Transit District 3 said they had not been contacted by police leadership, then promptly hung up.

The Mayor's Office has referred questions to the NYPD.

Dr. Alexandra Moffett-Bateau, a Harlem resident who first noticed the transit cops' display, said the symbol becomes especially menacing when hung inside a subway station primarily used by black New Yorkers.

"With the increase of police presence in the subways, flags like these only function to make black and brown people feel less safe taking public transportation," Moffett-Bateau, a political science professor at John Jay College, wrote in an email to Gothamist.

"The people in that neighborhood have a right to feel welcome and safe when they get on the train," she added. "The way it makes people of color, especially black folks like me, feel every time we come into the train station...they're trying to send a message to a predominantly black population."

The police flag was accompanied by a “thin red line” flag, which is meant to show support for firefighters. Federal law prohibits any display of the American flag that with a "mark, insignia, letter, word, figure, design, picture, or drawing of any nature."

During a press conference on Monday about the snow storm, Mayor Bill de Blasio brushed off questions about whether it's appropriate for the NYPD to fly the thin blue line flag on government property. "Is that about?" the mayor asked asked, trailing off. "We'll happily talk about those things another time."

Later in the day, the mayor presided over the swearing in of new NYPD Commissioner Dermot Shea, where he sharply criticized all those who would question the department's progress. "To the doubting Thomases, to the naysayers, if you doubt, then you don't truly respect the NYPD," the mayor declared.

Jeffrey Fagan, a law professor at Columbia University who specializes in police accountability and criminal law, said he was not surprised by Mayor de Blasio's silence on the flags.

"The Mayor is still the lapdog of the police unions," said Fagan. "It's disgraceful, and insulting to the people who voted for him."

Additional reporting by Gwynne Hogan.