Red shirts killed Col Romklao Thuwatham during violent clashes in April 2010, Department of Special Investigation chief Tarit Pengdith revealed yesterday.
Mr Tarit said United Front for Democracy against Dictatorship supporters had been responsible for the murder of Col Romklao,
"The DSI has continued to investigate this case, and we insist this is what happened," said Mr Tarit.
Col Romklao, from the Prachin Buri-based 2nd Infantry Division, was killed by a grenade blast on April 10, 2010 during an attempt by his unit to retake an area at Kok Wua intersection on Ratchadamnoen Klang Avenue which had been occupied by red shirts.
Col Romklao was among six soldiers and 20 protesters killed in the clashes.
Since his death, Col Romklao's wife, Nicha, has put pressure on the authorities, including the DSI, to seek justice for her husband.
Yesterday was the first time the DSI chief has clearly said that red shirts were behind the death of the colonel.
Mr Tarit earlier said his agency had found little evidence to support a theory that "men in black" killed Col Romklao at the rally.
He said the DSI has contacted Mrs Nicha for more information as the DSI needs more solid evidence to support its investigation report so the Criminal Court would not dismiss the case.
It is difficult for law enforcement officers to identify offenders during riots and the same goes for authorities in several other countries, he added.
Mr Tarit also said after the political violence in 2010 ended, he had proposed to the Centre for the Resolution of the Emergency Situation (CRES), which was established by the then Abhisit Vejjajiva government, that an amnesty law be enacted but almost all of the CRES members dismissed the idea.
He said divisions in society have become wider but it is not too late to solve the problems.
"The best way is for all parties to forgive each other," said Mr Tarit.