Demonstrations have now entered a fourth day after three nights of arrests and scuffles with police. More than 120 people were arrested late on Sunday, when police in riot gear used pepper spray and detained activists who defied orders to disperse following larger, peaceful protests.
A St. Louis Post-Dispatch photojournalist, David Carson, tweeted that he and others heard some officers chant, "Whose streets? Our streets," commandeering a refrain used by the protesters themselves.
A grainy video circulating online shows a group of officers and the chant can be heard. Carson said he spoke to a police commander at the scene who said any such chant would be unacceptable and he would deal with it.
The protests followed a judge's ruling on Friday finding the former officer, Jason Stockley, 36, not guilty of first-degree murder in the 2011 shooting death of Anthony Lamar Smith, 24.
The clashes evoked memories of riots following the 2014 shooting of a black teenager by a white officer in nearby Ferguson.
"The Department is aware of the video circulating on social media, and is reviewing the footage," police spokeswoman Schron Jackson wrote in an email to Reuters. "We hold our officers to the highest standards of professionalism and any officer not meeting those standards will be held accountable."
Such actions could only inflame tensions, said Nicolle Barton, executive director of the St. Louis police civilian oversight board, which is also investigating. "Certainly we do not want that to be taking place," she said.