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

Lawyer Tahir Elci was fatally shot in the head after 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, who are not in detention, attended the proceedings through video conference. In the tense hearing, the Elci family's lawyers questioned the impartiality of the court committee and asked them to recuse themselves.

At the time Elci, an ethnic Kurd, was killed 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. The hostilities came after a two-and-a-half-year long ceasefire collapsed, reigniting the conflict that began in 1984.

At the same time as Elci was making the press statement, two suspected PKK militants shot and killed two police officers in a nearby street and ran toward the street where the lawyer was speaking, exchanging gunfire with other police. Turkish officials previously said Elci got caught in the crossfire.

Research group Forensic Architecture, which published a detailed analysis of Elci’s killing, said the slayings of the police officers and of 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.

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 all three police officers as well as both militants had fired their guns.

Elci, 49, was the head of the Diyarbakir Bar Association. 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.

Elci worked as a human rights lawyer in southeastern Turkey, handling cases involving enforced disappearances and killings by security forces during the 1990s when the conflict between Turkey and the PKK was at its bloodiest.

Elci himself was a victim of torture in 1993 and won a case against Turkey a decade later at the European Court of Human Rights, which ruled Turkey violated the convention's articles on the prohibition of torture, right to liberty and security and the right to respect for private and family life.

Amnesty International's Turkey campaigner Milena Buyum said: "“It is a bitter irony that Tahir Elci's life was cut short by the very violence he was campaigning to end. Justice for Tahir Elci would be a glimmer of hope in a country where impunity is sadly endemic.”

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. The police officers who are now on trial were then only questioned as witnesses.

“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.”

The court adjourned until March 3.