Turkish and European lawmakers have called for transparency in the trial of 19 people accused of murdering Turkish- Armenian journalist Hrant Dink in 2007.
A statement by eight MPs, as well as lawyers and academics, demands a full investigation into claims that security officials colluded in Mr Dink's murder.
The statement came as a court in Istanbul convened for the third hearing in the case that shocked the nation.
Among the 19 defendants is a teenager accused of carrying out the killing.
The trial, which began in July, is being held behind closed doors because the alleged gunman, Ogun Samast, is 17 years old.
Mr Dink campaigned for his country to confront the mass killings of Armenians by Ottoman Turks during World War I.
Dozens of riot police were on duty as the latest court hearing in the murder of one of Turkey's most prominent journalists resumed on Monday, the BBC's Sarah Rainsford in Istanbul says.
Outside the court, supporters read the statement signed by the MPs and lawyers who believed the real forces behind Mr Dink's killing were still at large.
They demanded a full investigation into evidence that security officials colluded in his murder, calling that the ultimate test of Turkey's democracy and the rule of law.
"It's clear that police officers and security services knew about these plans of these guys but they didn't act," Dutch MEP Joost Lagendijk, who was present in the courtroom, told the BBC.
"Or some of them were probably actively involved in making the planning. All of these things should be dealt with in this court case, and if it doesn't happen it will leave a very dirty stain on Turkey's image," Mr Lagendijk said.
He also called on Turkey to move quickly to amend the controversial Article 301 of the penal code on insulting Turkishness.
Many believe Mr Dink's trial on this charge singled him out as a target for nationalists, our correspondent says.
Mr Dink's supporters also hope that last month's arrest of a suspected ultra-nationalist gang with alleged links to the security forces might help uncover those who plotted to kill the journalist, she adds.