ISTANBUL (AP) — Three police officers went on trial Wednesday in southeastern Turkey for the killing of a prominent lawyer and human rights defender nearly five years ago.

Lawyer Tahir Elci was shot and killed while making a press statement on the destruction of a historic mosque in the Sur district of Diyarbakir province. The officers are charged with “causing death by foreseeable negligence” in Elci’s Nov. 28, 2015 death and face possible prison sentences of two to six years in prison.

The police officers are attending the proceedings through video conference and are not in detention.

At the time, Sur and other areas in the southeast had seen intense clashes between Turkish security forces and members of the outlawed Kurdistan Workers’ Party, or PKK, and round-the-clock curfews. Two PKK militants shot and killed two police officers in a nearby street and ran toward the streets where the news conference was held, exchanging gunfire with other police officers.

Turkish officials previously said Elci got caught in the crossfire.

A fourth suspect on trial, an alleged member of the PKK who is not in custody, is accused of the intentional killing of the two police officers, the foreseeable intentional killing of Elci and “disrupting the unity and territorial integrity of the state.”

The 40-page indictment said the bullet that killed Elci was never found and therefore the gun could not be identified. It said the three police officers as well as two militants had fired their guns.

Elci, 49, was the head of the Diyabakir bar association and a human rights activist. He received death threats and was to go on trial for “terror propaganda” after calling the PKK an armed political movement rather than a terrorist organization during a television program.

Research group Forensic Architecture, which published a detailed analysis of Elci’s killing, said the 2015 slayings of the police officers and Elci should be considered separate incidents.

The research group, based at the University of London, said its investigation revealed that the shot that killed Elci could have only come from one of the police officers.

Human Rights Watch said there were “huge obstacles” to an effective investigation, including the alleged failure by investigators to collect evidence at the crime scene or to inspect the officers' firearms.

“For five years, the family and friends of Tahir Elci have pushed for an effective investigation of his killing and for his killers to be brought to justice,” Human Rights Watch deputy program director Tom Porteous said. “Many in the human rights movement in Turkey and internationally will be focused on whether the conduct of the trial is designed to reveal the full circumstances of Elci's killing or instead to try to exonerate the police at all cost.”