An explosion last weekend at an Iraqi military base hosting former pro-Iran paramilitaries was caused by munitions stored on-site, an investigative committee said Tuesday, ruling out an air strike.
The overnight Friday to Saturday blast at the Kalsu base in Babylon province killed one person and wounded eight others.
Officials had initially blamed an "aerial bombing", but the authorities have now dismissed the possibility of a drone or air strike.
The base housed "barracks and arms depots" belonging to the Iraqi army, the interior ministry and the Popular Mobilisation Forces (PMF), or Hashd al-Shaabi, the committee said in a statement.
"The explosion created a large, irregularly shaped crater at the site of the incident, (which was) used to store materiel, missiles and explosive materials", it said.
The committee, which conducted soil and material analyses at the site, added that "the size of the crater confirms there was a very strong explosion of weaponry and high explosive materials present at the site".
The analysis allowed the investigators to identify the presence of "three materials used in the manufacture of explosives and missiles - TNT, ammonium nitrate and dibutyl phthalate".
The committee said the intensity of the explosion and the size of various armaments thrown into the air by it "could not have been caused by one or multiple airborne missiles under any circumstances".
The investigators also pointed to reports by Iraq's air force command, which said there were no combat aircraft or drones flying in the province at the time of the attack.
CENTCOM, the US military command in the region, denied any involvement in the explosion.
When asked by AFP, the Israeli military refused to comment.
The PMF is an integral part of the Iraqi security forces operating under the authority of the prime minister.
But it encompasses multiple armed factions, some of which over recent months carried out dozens of attacks in Iraq and Syria against US soldiers deployed as part of an international anti-Islamic State group coalition.