BANGKOK (Reuters) - Thai authorities pressed for a new investigation on Friday into the death of Reuters cameraman Hiro Muramoto and 12 other civilians killed during political unrest last year, and said troops may have had a hand in the shootings.

The statement by the Department of Special Investigation (DSI) marked a dramatic reversal from February when the agency's chief concluded the 43-year-old Japanese journalist and others could not have been shot by soldiers.

Muramoto was killed by a high-velocity bullet wound to the chest while covering clashes between "red shirt" protesters and troops in Bangkok's old quarter on April 10. He was among 25 people, including soldiers, who died that night.

Mysterious gunmen clad in black were mingling with the red-shirted protesters and firing at troops.

Witness accounts in a preliminary DSI investigation seen by Reuters in December said the fatal shot came from the direction of troops. A witness was quoted as saying he saw "a flash from a gun barrel of a soldier," then watched Muramoto fall after he was shot in the chest while filming the security forces.

But on February 27, DSI Director-General Tharit Pengdit issued a new statement that contradicted the preliminary report, saying the bullet came from an AK-47 assault rifle, which did not match weapons used by soldiers in the area that day.

The Bangkok Post reported that, before Tharit made the claim soldiers were not involved, the army chief of staff had paid him a visit "to complain about an initial department finding. The DSI denied the report.

It was unclear what caused the DSI chief to change position again and assert soldiers may have played a role in civilian deaths -- an extraordinarily sensitive issue in Thailand where the military is extremely powerful and deeply politicised.

"We want the court to be the one who investigates this so that the result can be accepted by all," Tharit told Reuters on Friday. "The DSI has insisted from the beginning that we found soldiers may have been involved in the deaths."

CHANGE OF GOVERNMENT

One crucial difference between now and February is a change of government and the rise of political leaders with ties to the red-shirt protest movement.

A party backed red-shirt protesters won parliamentary elections by a landslide in July. Their leader, Prime Minister Yingluck Shinawatra, is the sister of deposed premier Thaksin Shinawatra, the figurehead of the red-shirt protest movement.

Yingluck's government has ushered in a series of changes to senior positions in several ministries and key institutions, including plans to change the chief of Bangkok's police.

Tharit said his agency would send on Monday the cases of the 13 civilians killed to police which would submit the findings to prosecutors, who would then bring the cases to Thailand's Criminal Court for a final investigation.

Deputy Prime Minister Chalerm Yoobamrung told reporters on Friday that the deaths of the 13 were "unnatural" and that it was unclear whether troops were involved.

"I want this to be very straightforward and everything is done under the rules of law," Charlerm said.

Muramoto, was based in Tokyo and had come to Bangkok to help cover the anti-government red shirt protests that lasted from March to mid-May last year. A total 91 people were killed during the unrest, including an Italian photojournalist and several medical workers.

The DSI has identified the cases of Muramoto and 12 other civilians, killed over the course of the unrest, for its initial investigation.

Then Prime Minister Abhisit Vejjajiva, whose government was backed by senior generals and the royalist establishment, repeatedly blamed the black-clad gunmen for civilian deaths despite witness accounts of soldiers opening fire with live ammunition on protesters.

(Additional reporting by Pracha Hariraksapitak and Ploy Ten Kate; Writing by Jason Szep; Editing by Robert Birsel)