Police Colonel Suebsak Pansura, who is heading a team investigating the case, said they had questioned 47 witnesses and experts over the death of Fabio Polenghi and gathered evidence to submit to prosecutors.
"The conclusion found that the cause of his death was believed to have been a gunshot from the authorities on duty," he told Bangkok's Criminal Court on the opening day of the inquest.
Polenghi was shot and killed on May 19 2010, the day when soldiers firing live ammunition stormed the anti-government "Red Shirt" protest movement's sprawling rally base in the centre of Bangkok.
Police could not find the bullet which fatally wounded him in the heart, but experts said he was shot by a high-velocity gun. The inquest will attempt to ascertain who was responsible.
Polenghi, 48, was working as a freelance photographer covering the protests in which tens of thousands of Red Shirts brought central Bangkok to a standstill for two months with demands for snap elections.
Street battles between soldiers with rifles and mostly unarmed protesters claimed more than 90 lives and left nearly 1,900 people injured, mainly civilians.
The kingdom, which remains deeply divided by the bloodshed, now has a new government allied to the Red Shirts' hero, fugitive former leader Thaksin Shinawatra, whose sister Yingluck is prime minister.
No soldiers or officials have been prosecuted in connection with the deaths during the unrest, prompting anger from relatives and rights groups, who say those responsible are being protected by a culture of impunity in Thailand.
Yingluck's government said in November there was clear evidence that troops were responsible for the death of another journalist during the unrest, Japanese cameraman Hiroyuki Muramoto of the Thomson Reuters news agency.