MEXICO CITY (AP) — An explosion occurred outside Mexico’s Attorney General’s Office on Thursday, injuring police as protesters demonstrating ahead of the anniversary of the 2014 disappearance of 43 students clashed with officers clad in riot gear.

Those injured by the explosion were loaded onto ambulances. Broken glass and blood were visible.

Members of a bomb squad cordoned off the area. One undetonated object that an explosives technician recovered appeared to be a small pipe bomb — a tube with two capped ends.

Mexico City’s police department said that 11 police officers were injured by shrapnel from fireworks and some suffered bruises. They were all taken to hospitals and the injuries were not considered life threatening.

The protest was just one of a host of activities planned in advance of Monday’s 8th anniversary of the students’ disappearances. Protests that includes relatives of the disappeared students have usually remained peaceful.

Thursday’s demonstration started that way too, with chants and speeches. Most of the protesters boarded buses and left before a small group that stayed behind clashed with police.

Some masked protesters threw rocks and launched bottle rockets into police lines. Others spray painted areas around the building with demands for the missing students’ safe return.

The police bunched together, crouching below their plastic shields and were engulfed in smoke.

“I was in the entrance to my store when four bombs went off like bottle rockets which is what they launched at the Attorney General’s Office, toward the windows,” said 19-year-old Jose Rivera Cruz, who sells clothing to one side of the office. “There was smoke and they closed the metro bus station (across the street). And most of the police were running and trying to get to the patrol cars and the ambulances.”

As more police arrived to help the injured and secure the area, the protesters left, he said.

On Sept. 26, 2014, local police in Iguala, Guerrero abducted 43 students from a radical teachers’ college. They were allegedly turned over to a drug gang and never seen again. Three victims were later identified by burned bone fragments.

Last month, Interior Undersecretary Alejandro Encinas, who leads a truth commission investigating the case, called it a “state crime” and directly implicated the military, among other state actors including local and state police.

Former Attorney General Jesus Murillo Karam, who oversaw the original investigation into the disappearances, was arrested last month on charges of torture, official misconduct and forced disappearance. Last week, Mexico arrested a retired general, who had been in charge of the local army base in Iguala when the abductions occurred.

Dozens of student protesters arrived at the Attorney General’s Office aboard buses Thursday morning. Police with helmets and riot shields formed several lines of defense in front the entrances.

On Wednesday, activists had vandalized the exterior of Israel’s embassy in Mexico City. Mexico is seeking the extradition from Israel of another key figure in the investigation of the students’ disappearances.

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