In the store were missiles, mortars, grenades and hundreds of thousands of bullets.
The US guard captured stores of Iraqi weapons and ammo around several sites in Baghdad. This one was in a neighbourhood in southern Baghdad.
But many Iraqis angry weapons were being kept so close to houses and hundreds started street protests against the US.
Many people who live near the area have been very badly hurt, most suffering from bad burns. At least four houses were destroyed.
There are still lots of injured people buried in rubble.
US officials are now investigating what exactly happened.