The High Court in the Solomon Islands capital, Honiara, has heard that militants shot two clerical brothers from an Anglican order beside an open grave.
Prosecutors allege four other Melanesian Brothers were killed in April, 2003, by four members of the former rebel group, the Guadalcanal Liberation Front.
Ronnie Cawa, Joses Kejoa and Owen Isa, have pleaded not guilty to murder charges in relation to the six deaths.
A juvenile only identified as 'K' also faces a murder charge and will be tried separately.
The trial judge, Justice Sekove Naqiolevu, heard that the gunshot injuries on five of the bodies were consistent with the accounts given by three accused men who are standing trial for murder.
Front leader Harold Keke was absent at the time of the killings and has not been charged.
He and Cawa were jailed for life in March for the murder of cabinet minister and Catholic priest Father Augustine Geve on August 20, 2002.
Cooper told the court the six brothers had gone to Guadalcanal's remote Weather Coast to inquire into the whereabouts of a seventh brother, Nathaniel Sado, who had disappeared in February, 2003, the Solomon Star reported today.
The Anglican brotherhood believed he might be held hostage by militants but he too had been killed.
Cooper outlined police interviews with militants which showed they believed the government had sent the Melanesian Brothers on an information-gathering mission to find where Keke was and how many guns the front had.
Cawa had ordered the brothers to be searched but when they resisted he ordered them killed, Cooper said.
Brothers Reuben Lindsay and Francis Tofi were shot by Cawa with a high-powered SR88 assault rifle.
Alfred Hill was accidentally shot in the arm and was then bashed to death, with Isa participating in that beating, the prosecutor said.
After the brothers' deaths, the three other brothers were held captive overnight but when they tried to escape, Cawa also ordered them killed, he said.
The next day Brothers Tony Sirihi, Patterson Gatu and Ini Paratabatu were accused of being spies and after being interrogated were taken to a freshly-dug grave and shot, the court heard.
The prosecution alleges Gatu was shot by the teenager 'K' and he and Cawa have been charged with his murder while Kejoa shot Paratabatu.
The trial continues.
The communal tensions in the Solomon Islands centred on conflict between the Guale people of Guadalcanal and migrants from the island of Malaita.
The arrival of the Australian-led Regional Assistance Mission to Solomon Islands in mid-2003 led to a restoration of law and order and the arrest of many former militants now facing a series of tension trials in the courts.