This image has the Mexican government embroiled in another deadly scandal

The Mexican government has another scandal on its hands after the Associated Press published a photograph on Sunday of what appears to be a federal policeman firing an assault rifle at protesters.

On Sunday hundreds of cops in riot gear clashed with protesters from a teachers’ union known as the National Coordinator of Education Workers (CNTE). The demonstrators, who blocked a federal highway in the southern state of Oaxaca, were reportedly burning vehicles and throwing stones and molotov cocktails at police.

The clash, which resulted in at least six dead and more than 100 injured, was the latest chapter in a nationwide protest by a radical teachers’ union that is rejecting President Enrique Peña Nieto’s education reform, signed into law in 2013. The reforms call for mandatory teacher evaluations to improve education standards.

The protests have been disruptive and unpopular, and until last weekend the government seemed to be winning the battle in the court of public opinion. But that changed on Sunday, when an AP image contradicted the government’s claim that cops were not using guns in their efforts to quell the riots.

In a press release Mexico’s Federal Police said the images of federal cops “using firearms against protesters” are “totally false.” Another press release insisted that the police officers responding to the protests were “unarmed.”

But Mexico’s Federal Police Commissioner acknowledged during an interview Sunday night with Mexican media outlet Animal Político that firearms were used at the end of the clash, when a different group of police arrived as backup. The police commissioner blamed the violence on infiltrators, who he claims fired shots at police and civilians.

Mexico’s Federal Police claim at least three cops were injured by firearms.

What happened on Sunday is still under investigation, but the federal police’s backtracking about the use of firearms has left many skeptical of the government’s version of events. Many are taking to social media to condemn the violence and criticize traditional media for not extensively covering the bloody incident.