The Department of Special Investigation is waiting for a copy of a court ruling on the 2010 death of a third red-shirt protester to consider whether to proceed with it, DSI chief Tarit Pengdit said.

The South Bangkok Criminal Court's  ruled on Monday morning that Chartchai Chaolao, a taxi driver from Surin province, was hit by a bullet fired from a line established by security forces on Rama IV Road on May 13, 2010.

But it was not known who fired the fatal shot, the court said in the ruling requested by the DSI.

Mr Tarit said the DSI, prosecution and police investigators would now consider whether to proceed with the case because the taxi driver did not die of natural causes but his death was caused by other persons.

Evidence showed that Chartchai was struck in the forehead by a .223-calibre or 5.56mm bullet that passed through his brain and shattered his skull.

The death occurred on May 13, 2010 in front of Krisna Marketing Co on Rama IV road opposite Lumpini Park.

The trajectory of the bullet indicated it came from a line of soldiers deployed along Rama IV road. However, it was not clear who fired the shot that killed Chartchai, the court said.

The ruling is the third of this kind, following rulings in the cases of red-shirt protesters Phan Khamkong, a taxi driver, and Channarong Polsricha.

In the two earlier cases, the Criminal Court ruled that the fatal shots were fired by soldiers performing security duty as ordered by the Centre for the Resolution of the Emergency Situation (CRES).

The ruling in the case of Phan Khamkong led to the indictment of former prime minister Abhisit Vejjajiva and his deputy Suthep Thaugsuban, former director of the CRES, on charges of murder brought by the DSI.

Mr Tarit said the DSI had to treat riot cases separately because each involved different circumstances and details. It was not as iff the deaths of all 98 people killed in the riots were caused by orders of the CRES, he said.

A case that involved a CRES order must unavoidably be proceeded with, Mr Tarit added.

The DSI chief also called on members of the Democrat Party not to impede his agency from doing its duty.

The Democrats have claimed that the decision to bring charges against Mr Abhisit and Mr Suthep appears politically motivated. However, Mr Abhisit has said that justice must be allowed to take its course in the courts and he would abide by any decision.