Observers claim that the fairness of an ongoing court martial of a group of elite soldiers who gunned down four detainees in a police jail in Yogyakarta in March is being compromised by the intimidation of prosecutors, judges and witnesses.

Teguh Soedarsono, a retired police general and senior official at the Witness and Victim Protection Agency (LPSK), said on Tuesday that the scope in which the prosecutors could present their case had narrowed as a result of the intimidation, making it less likely that they would be able to make a fair prosecution against the soldiers.

"The ruling from the judges will also be less than reasonable or just [under these circumstances]," he said as quoted by Tempo.co, adding that he could not see how a fair legal outcome could emerge from this.

Teguh said the intimidation came from supporters of the 12 soldiers standing trial. He said they sometimes locked the doors of the military tribunal building in Yogyakarta while prosecutors were presenting their case, and threatened people in the spectators' gallery.

"Where is the sense of comfort and security in the court when even the prosecutors and judges feel scared, not to mention the witnesses?" he said.

He added he found it disturbing that this was allowed to happen at all, given that the trial was meant to be open and that there were observers in attendance from the Supreme Court and the Judicial Commission.

Suprapto, a criminologist at Yogyakarta's Gadjah Mada University, said the soldiers' supporters had also put up banners throughout the city, including outside the military prosecutors' office, praising the defendants for their actions.

He warned that if the intimidation led to the soldiers being acquitted, then it would set a bad precedent for military accountability.

The 12 soldiers from the Army Special Forces (Kopassus) are being tried for storming the Cebongan detention center in Yogyakarta's Sleman district on March 23 and killing four detainees suspected of having murdered a Kopassus member four days earlier.

The military has repeatedly justified the summary executions as having been committed out of the soldiers' "esprit de corps," or solidarity for a fallen comrade.

In the trial, however, the soldiers claimed they acted in self-defense when they shot at the victims who were locked behind bars.

Those killed – Hendrik Angel Sahetapi, Yohanes Juan Manbait, Gameliel Yermianto Rohi Riwu and Adrianus Candra Galaja – were in jail over the death of Kopassus soldier Heru Santosa.