By Roland Buerk
BBC News, Colombo
A panel set up to monitor a Sri Lankan probe into human rights abuses has warned the process will end in failure unless changes are made.
The aid workers were found shot dead in Muttur in the north-east
The International Independent Group of Eminent Persons (IIGEP) is worried about the involvement of the attorney general's department in the inquiry.
It has warned there are conflicts of interest by its participation.
The government has come under pressure over human rights as the country had slid back into civil war.
'Lack of impartiality'
The report from international experts comes as Sri Lanka's President Mahinda Rajapaksa is in Geneva defending the country's human rights record.
The President's Commission was set up to look into a number of incidents that were blamed on the security forces and the Tamil Tiger rebels.
Monitors also looking into alleged Tamil Tiger attacks
It includes the assassinations of politicians, and the killing of 17 workers from the French charity Action Against Hunger, which Nordic ceasefire monitors blamed on the military.
As part of efforts to make the inquiry transparent the international community nominated a group of experts to observe the process.
In their second report they have said they are concerned about the involvement of lawyers from the attorney general's department.
The experts said they had "observed examples of a lack of impartiality", with counsel from the department stating "as fact matters which are controversial to the case".
The panel said the probe would end in failure unless independent lawyers are brought in.
The experts have also said they are concerned about the lack of an adequate witness protection programme, particularly amid many reports of abductions in the country.
The President's Commission has rejected the criticism from the experts saying it is satisfied the probe will yield results.
It also called for at least one member of the international panel to be in Sri Lanka to observe the proceedings.
Meanwhile Amnesty International has urged the president to ask the United Nations to set up a mission to monitor human rights in the country.
Ministers have previously rejected such a move, saying it would infringe on Sri Lanka's sovereignty.