The Department of Special Investigation (DSI) needs a complete overhaul to counter its growing image as a dysfunctional arm of justice, starting with the exit of its chief due to his failure to fulfil public expectations, human-rights groups said yesterday.

DSI director-general Sombat Amornviwat should quit because he was not competent to bring about real progress in three high-profile cases involving the deaths and disappearances of human-rights defenders and environmentalists, the groups said.

Muslim lawyer Somchai Neelaphaijit was abducted in March 2004 and is presumed dead, community-rights activist Charoen Wat-aksorn was gunned down in June 2004, and environmentalist monk Phra Supoj Suwajo was murdered in June 2005, the group said.

"The failure of the DSI to suppress organised crime and terrorist acts inevitably erodes public confidence in the justice system," the groups said in an open letter to caretaker Justice Minister Chitchai Wannasathit.

"That may lead to ever more unrest and violence in society, for which the government cannot shirk responsibility."

The DSI is a department of the ministry.

The human-rights advocates see a common thread in the three unresolved cases in that the victims got in the way of parties with vested interests, such as government officials, politicians and influential figures.

Somchai's case saw only one of the defendants, all police officers, convicted of coercion while the four accomplices were acquitted.

"Above all the DSI has never been able to identify officially the status of the missing lawyer [despite] the serious attention [given by the media and the public]," the statement said.

The groups, including family members of those killed, among them Somchai's wife Angkhana, said that in Charoen's case the DSI "rushed to conclude that it was merely due to a personal conflict, and no further investigation was conducted to find the masterminds, despite a litany of material evidence and witnesses pointing to the real cause of the murder".

Charoen was the leader of a grass-roots campaign against a proposed coal-fired power plant and land-grabbing in Prachuap Khiri Khan province.

Although Phra Supoj's case was supposedly recently revived, the DSI said it had been unable to come up with any leads, suspects, witnesses, weapons or any material evidence, the groups said.

They proposed that the DSI, which was created in 2004, should select a new chief through a transparent process so that it could act independently.