Updated - Three far-left kidnappers who held an Istanbul prosecutor hostage today were shot and killed in a raid by special forces, while the prosecutor himself was seriously injured. 

Earlier, the Revolutionary People's Liberation Party-Front (DHKP-C) published a picture of the prosecutor with a gun to his head and said it would kill him .

Istanbul police chief Selami Altinok said negotiators had been talking to the hostage-takers.

The prosecutor, Mehmet Selim Kiraz, was leading an investigation into the death last March of 15-year-old Berkin Elvan, who died after nine months in a coma from a head wound sustained in anti-government protests.

The DHKP-C said on its website it wanted the police officer it blames for Elvan's death to confess on television, the officers involved to be tried in "people's courts", and charges against those who attended protests for Elvan to be dropped.

Witnesses said they heard gunshots as the hostage-takers entered the building.

"We were on the sixth floor. A black-haired man wearing a suit entered the prosecutor's room and fired a gun three times," Mehmet Hasan Kaplan, who works in the building, told Reuters, adding that the attackers also claimed to have explosives.

In a brief video message on a widely-followed Twitter account describing itself as that of Elvan's family, the boy's father appeared to call on the group not to harm the prosecutor.

"We want justice. We don't want anyone to shed even a drop of blood. We don't want other mothers to cry," Sami Elvan said. 

The United States, European Union and Turkey list the DHKP-C as a terrorist organisation. It was behind a suicide bombing at the U.S. Embassy in 2013. In 2001, two policemen and an Australian tourist died in a DHKP-C attack in central Istanbul.

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