The blast – which destroyed an entire truck of munitions and levelled buildings – also wounded several soldiers and at least one child in rural Kampong Speu province on Saturday.
The defence ministry said on Thursday that investigators believed the heatwave played a role in the old weapons detonating.
“The incident of the ammunition explosion on April 27, 2024 … was a technical issue because the weapons are old, faulty, and the hot weather,” the ministry said in a statement.
It did not explain the specific issue or how the heat may have contributed to the explosion.
The ministry also rejected suggestions that the blast was down to rebellious soldiers or an act of terrorism.
‘Very dangerous’: Thai heat index tops 52 degrees amid Philippines’ power woes
‘Very dangerous’: Thai heat index tops 52 degrees amid Philippines’ power woes
Deadly munitions accidents are not uncommon in Cambodia, which is awash with ammunition, mines and unexploded ordnance leftover from decades of civil conflict.
Such incidents are exacerbated by frequently lax safety standards.
Like much of South and Southeast Asia, Cambodia has been struggling with hot weather in recent weeks.
Authorities warned on Sunday that temperatures could reach 43 degrees Celsius (109.4 degrees Fahrenheit) in some areas, although rain and cooler weather were forecast in the coming days.
Following the blast west of the capital Phnom Penh, images posted on social media showed a destroyed one-story building wreathed in smoke, with residents of a nearby village also sharing images online of broken windows.
Other photos showed what appeared to be civilians, including a young child in a diaper, with cuts and gashes being treated in hospital.
An office building and nearby barracks were destroyed, and 25 nearby homes were also damaged.
Prime Minister Hun Manet said the families of those killed would receive roughly US$20,000 each, while injured soldiers would get US$5,000.